docs: docs-as-code documentation system for v0.9.1

[ikelaiah]

Summary

Introduces a complete docs-as-code documentation system for ThreadPool-FP (still v0.9.1). Reuses the proven lightweight, dependency-free static-documentation architecture used by ChronoKit-FP and StringKit-FP and adapts it to ThreadPool-FP's own identity.

What it does

• Curated information architecture (docs/layout.json + docs/versions.json): Getting Started, 12 Guides, Reference, and Internals navigation, with release notes / PR notes / archived plan / ADR kept as hidden-but-searchable pages.
• Homepage with project-specific hero, 6 navigation cards, and the existing SVG banner.
• Client-side search over titles, headings, prose, and API symbols (GlobalThreadPool, TSimpleThreadPool, IThreadPoolTask, SubmitRange, TryQueue, WaitForAll, Shutdown, ...), keyboard shortcut /.
• Versioned output at /threadpool-fp/0.9.1/ with /threadpool-fp/ redirect, version selector, light/dark themes, theme selector, mobile nav, TOC, breadcrumbs, prev/next pagination, copy code blocks, offline ZIP + SHA-256.
• Concurrency-accurate docs: guides/thread-safety.md and reference/contracts-and-limitations.md state exact guarantees (WaitForAll, cancellation, error containment, tombstones, deadlock guards, deadlines, backpressure) verified against source and the test suite.
• Recipes = real code: 12 compiled programs under examples/documentation/ with expected-output files; code blocks in docs/guides/recipes.md are checked byte-identical to their source by tools/check_docs.py, and test_docs_examples.py compiles and runs them (FPC 3.2.2).
• Validators + tests: build_docs.py, build_all_docs.py, check_docs.py, check_built_docs.py plus their test_*.py suites; the build fails on broken links, unsafe URLs, layout mismatches, or invalid metadata.
• GitHub Pages automation: .github/workflows/documentation.yml (targeted on releases / workflow dispatch) builds, validates, and deploys via current actions/configure-pages + deploy-pages; CI gains a Documentation checks job and always compiles/run the recipe examples on Windows and Linux.
• README / CONTRIBUTING / examples README link to the rendered site at https://ikelaiah.github.io/threadpool-fp/.

Verification

All green locally (expected to reproduce in CI):

• test_build_docs / test_build_all_docs / test_check_built_docs / test_check_docs unit suites
• check_docs.py + check_built_docs.py --site site
• test_docs_examples.py — 12 programs compiled with FPC 3.2.2 and run against expected output
• Full 36-page static site build + link/anchor/metadata validation

Manual step after merge

Enable Settings → Pages → Source = GitHub Actions so the provided workflow can publish.

Notes

• Current release remains 0.9.1; only versions whose tag contains the documentation source tree are exposed (an earlier tag cannot honestly host the new layout).
• Historical material (release notes, archived plan, PR notes, ADR) is preserved under docs/history/ and served as hidden, searchable pages — not primary navigation.