Issue 1920 - Adds missing doxygen links to RST

[Rohit-Kakodkar]

Description

Please describe the changes/features in this pull request.

• Several docs changes

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
• THE DOCUMENTATION BUILDS WITHOUT WARNINGS/ERRORS
• I have added labels to the PR (see right hand side of the PR page)
• 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

[Copilot]

Pull request overview

This PR updates the Sphinx/Doxygen API documentation pages to add missing namespace/function links and to include new API sections (chunk_face and constants) in the main specfem API toctree.

Changes:

• Updated several API RST pages to use fully-qualified symbol headings and/or add .. doxygennamespace:: blocks for better Doxygen cross-linking.
• Added new API documentation sections for specfem::constants and specfem::chunk_face.
• Wired the new sections into docs/sections/api/specfem/index.rst.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
docs/sections/api/specfem/utilities/strings.rst	Uses fully-qualified function headings for improved Doxygen cross-references.
docs/sections/api/specfem/units/conversions.rst	Converts the page to a direct unit_cast API entry.
docs/sections/api/specfem/tag_dispatch/index.rst	Adds doxygennamespace for specfem::tag_dispatch (desc-only).
docs/sections/api/specfem/io/backends/index.rst	Updates namespace naming to specfem::io_backends and adds doxygennamespace.
docs/sections/api/specfem/index.rst	Adds chunk_face and constants sections to the main API toctree.
docs/sections/api/specfem/constants/index.rst	New page documenting key constants via doxygenvariable.
docs/sections/api/specfem/chunk_face/index.rst	New chunk_face namespace landing page + toctree.
docs/sections/api/specfem/chunk_face/acceleration.rst	New page documenting specfem::chunk_face::acceleration.
docs/sections/api/specfem/chunk_face/velocity.rst	New page documenting specfem::chunk_face::velocity.
docs/sections/api/specfem/chunk_face/displacement.rst	New page documenting specfem::chunk_face::displacement.
docs/sections/api/specfem/chunk_face/chunk_face_index.rst	New page documenting specfem::chunk_face::Index.
docs/sections/api/specfem/chunk_face/nonconforming_interface.rst	New page documenting nonconforming-interface related chunk_face structs.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.