Add opinionated MkDocs + GitHub Pages docs tooling to template

[thibaudcolas]

Summary

Adds an opinionated documentation setup to the template, replicating the tooling from draftjs_exporter: MkDocs + Material for MkDocs + mkdocstrings, published to GitHub Pages.

Docs tooling:

• mkdocs.yml — Material theme, markdown extensions, mkdocstrings wired to src/ so the API reference is generated from docstrings, and mkdocs-llmstxt generating llms.txt / llms-full.txt.
• docs/ — index, getting started, usage, API + configuration reference, agent skills page, and contributing/documentation.md documenting the tooling evaluation (Sphinx vs Zensical vs MkDocs; Read the Docs vs GitHub Pages) against the agreed criteria (ease of use, docstring support, deployment fit, maturity).
• just docs-build (--strict) and just docs-serve recipes; CI builds docs strictly and deploys to GitHub Pages on pushes to main via actions/deploy-pages.

Agent skills scaffolding:

• Demo placeholder skill at src/my_project_name/.agents/skills/my-project-name/SKILL.md, with its description wired to the short-description placeholder.
• docs/hooks.py — on_post_build hook copying skills into .well-known/agent-skills/ and generating index.json + ai-catalog.json discovery catalogs. Host metadata (did:web, URN authority, URLs) is derived from site_url at build time, so it is correct in both the template and generated packages.
• include-hidden-files: true on the Pages upload so the .well-known/ files deploy.

Template mechanics:

• Derived __project_site_url / __project_org cookiecutter variables so the Pages URL resolves in generated packages; kebab-case placeholder directories renamed by the pre-prompt hook.
• docs/theme added to _copy_without_render so cookiecutter leaves the MkDocs Jinja override untouched.

Review fixes folded in (found while verifying this PR):

• The pre-prompt hook now also skips top-level git-ignored directories and root .agents/ when building the generated project. Previously, running cookiecutter . from any working checkout copied .venv/site/node_modules into the generated package — raw Jinja templates inside .venv then broke rendering with a cryptic TemplateSyntaxError — and the template's own authoring skills leaked into generated packages. Untracked (non-ignored) content is still copied, and there is a static fallback when git is unavailable (zip downloads).
• /site (docs build output) added to .stylelintignore / .prettierignore, so just lint keeps passing after running just docs-build.
• uv.lock regenerated on top of latest main (Python 3.10 drop, uv_build bump, lock maintenance), and docs/hooks.py updated for the newer Ruff (datetime.UTC).

Verification

• just lint, just docs-build (strict, 0 warnings), and pytest tests pass in this repo; just test's only failure is pytest collecting the untracked local/draftjs_exporter reference copy in this workspace (never present in CI).
• Generated a package with cookiecutter . --no-input from both a clean clone and a working checkout: placeholders resolve, exactly one demo skill is scaffolded at src/wagtail_cool_project/.agents/skills/wagtail-cool-project/, mkdocs build --strict passes, and the built site publishes correct .well-known/agent-skills/index.json, .well-known/ai-catalog.json and llms.txt (host did:web:org-name-or-username.github.io:wagtail-cool-project, description pulled from the cookiecutter variable).
• Ruff clean in both the template and the generated package.

[zerolab]

Why go for mkdocs instead of zensical when mkdocs v1.x is no longer maintained and v2 going in quite a different direction making it difficult to work with plugins from what I understand?