feat: add Doxygen documentation support

Add Doxygen integration for generating API documentation from source
code comments. This provides professional HTML documentation for
library users.

Changes:
- Add docs/Doxyfile.in: CMake-configurable Doxygen template
- Add docs/mainpage.dox: Documentation landing page
- Update CMakeLists.txt: Add BUILD_DOCS option and docs target
- Update header files with Doxygen-compatible comments:
  - include/profiler_manager.h
  - include/symbolize.h
  - include/web_server.h

Usage:
  cmake --preset=release -DBUILD_DOCS=ON
  cmake --build build --target docs

Output: build/docs/html/index.html

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: IronsDu