Domain documentation update

[Rohit-Kakodkar]

Description

Updates documentation for domain optimization. I've made several major changes on how domain is organized using sphinx.

1. Reorganized API documentation to use doxygenclass over doxygenfile - this improves readability and flexibilty in the the new docuementation
2. Added SPECFEM architecture documentation
3. Current change updates the name from SPECFEM Kokkos to SPECFEM++

Issue Number

If there is an issue created for these changes, link it here

Checklist

Please make sure to check developer documentation on specfem docs.

• I ran the code through pre-commit to check style
• My code passes all the integration tests
• I have added sufficient unittests to test my changes
• I have added/updated documentation for the changes I am proposing
• I have updated CMakeLists to ensure my code builds
• My code builds across all platforms