docs: add diagrams for course cert awarding and revocation

[APER-2551]

This PR adds some diagrams to help visualize the current certificate awarding and revocation process of the monolith.

lms/djangoapps/certificates/docs/diagrams/README.rst

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+Certificate management workflows are spread across a few different apps of edx-platform and can be hard to trace and
+understand. This folder contains high level visualizations to help maintainers understand how the Certificate awarding
+and revoking flows work, and how they interact with other pieces of the Open edX ecosystem (e.g. the Credentials IDA and
+the Event Bus).

lms/djangoapps/certificates/docs/diagrams/certificate_generation.dsl

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+/*
+ * This is a high level diagram visualizing how the system awards course and program certificates. It is written using
+ * Structurizr DSL (https://structurizr.org/).
+ */
+workspace {
+    model {
+        properties {
+            "structurizr.groupSeparator" "/"
+        }
+        event_bus = softwareSystem "Event Bus"
+
+        group "LMS" {
+            grades_app = softwareSystem "Grades Django app"
+            verify_student_app = softwareSystem "Verify Student app"
+            student_app = softwareSystem "Student app"
+            group "Certificates app" {
+                signal_handlers = softwareSystem "Certificates Signal Handlers"
+                generation_handler = softwareSystem "Certificates Generation Handler"
+                allowlist = softwareSystem "Certificate AllowList"
+            }
+            credentials_app = softwareSystem "Credentials Django app"
+            programs_app = softwareSystem "Programs Django App"
+            celery = softwareSystem "Celery"
+            database = softwareSystem "Database"
+        }
+
+        group "Credentials IDA" {
+            credentials_ida_consumer = softwareSystem "Credentials Event Bus Consumer"
+            credentials_api_app = softwareSystem "Credentials API Django App"
+            credentials_credentials_app = softwareSystem "Credentials Django App"
+        }
+
+        grades_app -> signal_handlers "Emits COURSE_GRADE_NOW_PASSED signal"
+        verify_student_app -> signal_handlers "Emits LEARNER_NOW_VERIFIED signal"
+        student_app -> signal_handlers "Emits ENROLLMENT_TRACK_UPDATED signal"
+        allowlist -> signal_handlers "Emits APPEND_CERTIFICATE_ALLOWLIST signal"
+        signal_handlers -> generation_handler "Invokes generate_allowlist_certificate()"
+        signal_handlers -> generation_handler "Invokes generate_regular_certificate()"
+        generation_handler -> celery "Enqueues generate_certificate_task task"
+        celery -> database "UPSERT certificate record to database"
+        database -> event_bus "Emits a CERTIFICATE_CREATED event"
+        database -> programs_app "Emits a COURSE_CERT_CHANGED signal"
+        credentials_ida_consumer -> event_bus "Listening for Certificate events"
+        database -> programs_app "If passing, emits a COURSE_CERT_AWARDED signal"
+        programs_app -> celery "Enqueue award_course_certificate task"
+        programs_app -> celery "Enqueue award_program_certificate task"
+        celery -> credentials_app "Process award_course_certificate task"
+        celery -> credentials_app "Process award_program_certificate task"
+        credentials_app -> credentials_api_app "POST request to award course credential"
+        credentials_app -> credentials_api_app "POST request to award program credential"
+        credentials_ida_consumer -> credentials_credentials_app "Award Certificate record"
+    }
+
+    views {
+        systemLandscape "SystemLandscape" {
+            include *
+            autolayout lr
+        }
+
+        styles {
+            element "Database" {
+                shape Cylinder
+            }
+        }
+    }
+}

lms/djangoapps/certificates/docs/diagrams/certificate_revocation.dsl

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+/*
+ * This is a high level diagram visualizing how the system revokes course and program certificates. It is written using
+ * Structurizr DSL (https://structurizr.org/).
+ */
+workspace {
+    model {
+        properties {
+            "structurizr.groupSeparator" "/"
+        }
+        event_bus = softwareSystem "Event Bus"
+
+        group "LMS" {
+            grades_app = softwareSystem "Grades Django app"
+            group "Certificates app" {
+                signal_handlers = softwareSystem "Certificates Signal Handlers"
+            }
+            credentials_app = softwareSystem "Credentials Django app"
+            programs_app = softwareSystem "Programs Django App"
+            celery = softwareSystem "Celery"
+            database = softwareSystem "Database"
+        }
+
+        group "Credentials IDA" {
+            credentials_ida_consumer = softwareSystem "Credentials Event Bus Consumer"
+            credentials_api_app = softwareSystem "Credentials API Django App"
+            credentials_credentials_app = softwareSystem "Credentials Django App"
+        }
+
+        grades_app -> signal_handlers "Emits COURSE_GRADE_NOW_FAILED signal"
+        signal_handlers -> database "Update certificate's status to NOT_PASSING"
+        database -> event_bus "Emits a CERTIFICATE_REVOKED event"
+        credentials_ida_consumer -> event_bus "Listening for Certificate events"
+        credentials_ida_consumer -> credentials_credentials_app "Revoke Certificate record"
+        database -> programs_app "On save(), emits a COURSE_CERT_CHANGED signal"
+        database -> programs_app "On save(), emits a COURSE_CERT_REVOKED signal"
+        // The following line is correct, an `award_course_certificate` task is queued whether we award or revoke a
+        // course certificate. It handles both operations.
+        programs_app -> celery "Enqueue award_course_certificate task"
+        programs_app -> celery "Enqueue revoke_program_certificate task"
+        celery -> credentials_app "Process award_course_certificate task"
+        celery -> credentials_app "Process revoke_program_certificate task"
+        credentials_app -> credentials_api_app "POST request to revoke course credential"
+        credentials_app -> credentials_api_app "POST request to revoke program credential"
+    }
+
+    views {
+        systemLandscape "SystemLandscape" {
+            include *
+            autolayout lr
+        }
+
+        styles {
+            element "Database" {
+                shape Cylinder
+            }
+        }
+    }
+}