docs: add generated reference site

[tetsuo-cpp]

Summary

• add an opt-in Sphinx/MyST/Furo reference site with a concise landing page and handwritten language and GPT-2 demo guides
• generate portable Forth dialect documentation directly from TableGen without Hugo-specific output
• generate a linked C++ API and source reference with Doxygen
• expose the complete site through the docs-warpforth CMake target and validate it in CI

Documentation remains disabled in normal builds unless WARPFORTH_ENABLE_DOCS=ON.

Validation

• cmake --build build-docs --target docs-warpforth
• cmake --build build-docs --target check-warpforth — 115/115 tests passed
• rendered home, language, dialect, demo, and C++ API pages inspected through the documentation portal
• PR build-and-test workflow passed before the squash