feature: add mkdocs-material documentation site with CI deployment

Generate comprehensive documentation for all 173 source units and
integrate mkdocs-material into the build toolchain. Documentation
covers kernel core, x86 architecture, drivers, applications, and
all subsystems with a tabbed navigation structure.

- Add mkdocs.yml with material theme, mermaid support, and tabbed nav
- Add doc/index.md landing page
- Generate doc/src/ markdown for every Pascal unit in src/
- Reorganize planning docs into doc/planning/
- Add compat, lvglh, and toolchain doc sections
- Update Dockerfile to install python3 and mkdocs-material (pinned <2)
- Replace pasdoc-based compile_docs.sh with mkdocs build --strict
- Add compile_docs.sh to the main build pipeline in compile.sh
- Add deploy-docs step in .drone.yml with host volume mount to nginx
- Add site/.gitkeep for build output directory

doc/toolchain/compile.md

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+# Placeholder

doc/toolchain/compile_checksum.md

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+# Placeholder

doc/toolchain/compile_docs.md

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+# compile_docs.sh
+
+Documentation site generator using MkDocs Material.
+
+## Overview
+
+Builds the static documentation website from the Markdown files in the `doc/` directory using mkdocs-material. Replaces the previous pasdoc-based HTML generation.
+
+## Dependencies
+
+- Python 3
+- `mkdocs-material` (installed automatically if missing)
+
+## Behavior
+
+1. Checks whether `mkdocs` is available on the `PATH`.
+2. If not found, installs `mkdocs-material` via pip.
+3. Runs `mkdocs build --strict` to generate the static site.
+4. Output is written to the `site/` directory at the project root.
+
+## Docker Usage
+
+The documentation site can also be built and served via Docker Compose:
+
+```bash
+# Live development server with hot-reload
+docker compose up docs
+
+# Static build only
+docker compose run docs build --strict
+```
+
+The development server listens on `http://localhost:8000`.
+
+## Configuration
+
+The site is configured by `mkdocs.yml` in the project root. The navigation structure mirrors the `doc/` directory hierarchy.

doc/toolchain/compile_finish.md

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doc/toolchain/compile_isogen.md

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+# Placeholder

doc/toolchain/compile_link.md

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+# Placeholder

doc/toolchain/compile_lvgl.md

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+# Placeholder

doc/toolchain/compile_sourcelist.md

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+# Placeholder

doc/toolchain/compile_sources.md

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+# Placeholder

doc/toolchain/compile_stub.md

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+# Placeholder

doc/toolchain/compile_sumgen.md

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+# Placeholder

doc/toolchain/compile_vergen.md

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+# Placeholder

doc/toolchain/compile_wasuro.md

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+# Placeholder

doc/toolchain/loc.md

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+# Placeholder