Title: Docs: coherent index structure + Python Client API link + safer Sphinx builds

Fixes: #

About

This PR reorganizes the documentation navigation, fixes incorrect/obsolete paths, and makes the Sphinx builds more deterministic.

Highlights

• Reworked docs/index.md into a coherent, multi-section tree:

  • Introduction: adds transition_from_airsim.md, system_specs.md, faq.md, license.md
  • Getting Started: fixes paths (development/use_source.md, development/use_prebuilt.md, etc.)
  • Configuration, Sensors, Controllers, Scene & Simulation: grouped by topic with existing pages wired into the tree
  • APIs & Programmatic Control: adds api.md and ros/ros.md
  • Python Client API: adds a toctree link to client_api/index.html (intended target of the Python client autodoc export)

• Removed/rewired stale references:

  • Dropped code_structure.md section
  • Replaced non-existent development/physics/unreal_physics.md and development/scene/sim_clock_development.md with the existing development/scene/sim_clock_internal.md

• CI improvement in .github/workflows/sphinx-docs.yml:

  • Clean build directories (rm -rf build) before make html for both the Python client docs and the main docs to avoid stale artifacts
  • Placeholder step to copy/export Python client docs into the main site (see “Follow-ups”)

How Has This Been Tested?

• Local (Ubuntu 22.04):

  • Ran make -C client/python/projectairsim/docs html to generate the Python client docs
  • Ran make -C docs html to build the main site
  • Verified the new navigation and that internal links resolve (quick manual pass)

• CI (GitHub Actions):

  • Workflow builds the Python client docs and main docs with a clean build each time
  • Verified the job succeeds without stale-build issues

• Result: “Python Client API” menu item points to client_api/index.html (when the client docs are exported/copied into docs/build/html/client_api).

Screenshots and videos (if appropriate):

• Included diffs of .github/workflows/sphinx-docs.yml and docs/index.md showing the changes in structure and build steps.

Follow-ups (separate PR or commit)

• Wire the “Copy Python client docs into main docs” step to one of these:

  • Export approach: build the client docs directly into docs/build/html/client_api (e.g., sphinx-build -b html client/python/projectairsim/docs docs/build/html/client_api)
  • Copy approach: build to the client folder first, then copy to docs/build/html/client_api

• Optionally add a linkcheck job (sphinx-build -b linkcheck) to catch broken links automatically.