Improve Doxygen

[pascalecu]

Summary

Integrates Doxygen documentation generation into the CMake build system and updates the documentation deployment workflow to use the generated CMake target.

Changes

• Added CMake-based Doxygen generation through a new docs build target.
• Converted Doxyfile into a configurable Doxyfile.in template using CMake variables:
  • Project name is populated from PROJECT_NAME.
  • Documentation output is placed inside the CMake build directory.
  • Source paths are resolved relative to the project source directory.
• Updated Doxygen configuration:
  • Added support for documenting the project README as the main page.
  • Enabled additional source extraction and warning checks.
  • Added custom documentation aliases (todo, invariant, pre, post, complexity).
  • Enabled source browsing, call graphs, references, and interactive SVG diagrams.
• Added strict out-of-source build enforcement:
  • Detects previous accidental in-source CMake build attempts.
  • Prevents configuring Azin directly inside the source tree.
• Updated GitHub Actions documentation deployment:
  • Builds documentation through CMake instead of invoking Doxygen directly.
  • Uses Ninja for faster CI builds.
  • Deploys generated documentation from the CMake build output directory.

Testing

Verified that documentation can be generated with:

cmake -S . -B build -G Ninja
cmake --build build --target docs

[yydev-official]

yay! doxygen documentation is done!
approved!

[spatulari]

lgtm