Add MkDocs documentation site with GitHub Pages deployment

[Copilot]

Adds a full MkDocs documentation site for the FOOL Compiler, configured to deploy automatically to GitHub Pages via GitHub Actions.

Documentation structure (docs/)

• Home – project overview, compiler phase table, quick links
• Getting Started – prerequisites, build commands, example programs
• Language Reference – grammar (FOOL.g4 annotated) and feature guide with code examples
• Architecture – one page per compiler phase: AST generation, symbol table, type checking, code generation (SVM assembly, AR layout, dispatch table)

Configuration

• mkdocs.yml – Material theme with dark/light toggle, navigation tabs, code copy, --strict build
• .github/workflows/docs.yml – builds on push to main and deploys via actions/deploy-pages; requires Settings → Pages → Source: GitHub Actions to be enabled in the repository

Housekeeping

• site/ added to .gitignore

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