docs: add generated documentation site

[tetsuo-cpp]

Summary

• add an opt-in Sphinx/MyST documentation site using LLVM's Furo theme
• stage README.md as the single-source home page and port the existing language and GPT-2 documentation
• generate the complete Forth dialect reference from TableGen and handwritten C++ API/source documentation with Doxygen
• expose one docs-warpforth CMake target and build it in CI

Validation

• cmake --build build-docs --target docs-warpforth
• cmake --build build-docs --target check-warpforth (115/115 tests passed)
• cmake --build build-docs --target check-format
• cmake --build build-docs --target check-clang-tidy
• browser verification of the home, language, dialect, demo, and C++ API pages

[tetsuo-cpp]

/gpu-test

[tetsuo-cpp]

/gpu-test