docs: Add documentation for motion planning

doc/07_planning/motion_planning.md

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+# Motion Planning
+
+**Summary:** [motion_planning.py](.../code/planning/local_planner/src/motion_planning.py):
+The motion planning is responsible for collecting all the speeds from the different components and choosing the optimal one to be fowarded into the acting.
+
+---
+
+## Author
+
+Julius Miller
+
+## Date
+
+17.12.2023
+
+## Prerequisite
+
+---
+<!-- TOC -->
+- [Motion Planning](#motion-planning)
+  - [Author](#author)
+  - [Date](#date)
+  - [Prerequisite](#prerequisite)
+  - [Description](#description)
+    - [Inputs](#inputs)
+    - [Outputs](#outputs)
+<!-- TOC -->
+
+---
+
+## Description
+
+This node currently gathers the behavior speed and the acc_speed and chooses the lower one to be forwarded to the acting.
+At the moment this is enough since the only present behaviors are Intersection, Lane Change and Cruise.
+
+When the Overtaking behavior will be added, choosing the minimum speed will not be sufficient.
+
+### Inputs
+
+This node subscribes to the following topics:
+
+- Current Behavior:
+  - `/paf/{role_name}/curr_behavior` (String)
+- ACC Velocity:
+  - `/paf/{role_name}/acc_velocity` (Float32)
+- Waypoint:
+  - `/paf/{role_name}/waypoint_distance` (Waypoint)
+- Lane Change:
+  - `/paf/{role_name}/lane_change_distance` (LaneChange)
+
+### Outputs
+
+This node publishes the following topics:
+
+- Target Velocity
+  - `/paf/{role_name}/target_velocity` (Float32)