Milestone m3: MkDocs Material toolchain, nav, and strict Pages build

[CooperBigFoot]

Integration PR for milestone m3 of the pyshed-release-and-docs-modernization campaign.

• mkdocs.yml: Material theme, mkdocstrings[python] in static mode against crates/python/python/pyshed/init.pyi (no compiled extension, no GDAL), full approved nav (Home / Quickstart / How it works / Guide / API Reference / Advanced / Credits & Citation), exclude_docs for the maintainer-facing docs/README.md.
• PEP 735 [dependency-groups] docs group + Documentation URL in crates/python/pyproject.toml (additive uv.lock re-lock).
• .github/workflows/docs.yml: separable build (uv run --only-group docs mkdocs build --strict + Pages artifact) and deploy (github-pages environment) jobs on push-main + workflow_dispatch.
• Root Makefile with docs / docs-serve targets; .venv/ ignore line; placeholder pages for all net-new nav entries; telemetry.md dead-link fix.

NOTE FOR MAINTAINER: the docs.yml deploy job will fail until GitHub Pages is enabled (source: GitHub Actions) and the github-pages environment exists — see RELEASING.md's setup checklist. The build job is green regardless.

Covers vision items 6-10; satisfies acceptance criteria 3 and 5. One squashed step commit (#15). Unblocks m4 and m5 (content).