docs: add MkDocs Material site + GitHub Pages deploy

[dustenhubbard]

Summary

Adds a MkDocs Material documentation site rendered from docs/, deployable to
GitHub Pages. This is additive: it does not touch the GitHub wiki or the
in-app Help "user guide" link (see Open decisions below).

What's here

• mkdocs.yml at the repo root: Material theme; site_name "PyReconstruct";
  repo_url / repo_name point at this fork; docs_dir: docs. Features:
  navigation.instant, navigation.top, navigation.sections, search.suggest,
  content.code.copy; light/dark palette toggle; the built-in search plugin.
• Nav surfacing the existing content:
  • User Guide renders docs/USER_GUIDE.md.
  • Performance (docs/performance.md) summarizes the fork's benchmark
    headline and links out to benchmarks/REPORT.md and the harness.
  • Developing with uv renders the existing docs/DEV_UV.md.
  • Contributing and Security are out-links to CONTRIBUTING.md and
    SECURITY.md (kept at the repo root, not duplicated into docs/).
• docs/index.md: a short landing page (what PyReconstruct is, an install
  pointer, and links into the guide), reusing the README's framing without
  duplicating it.
• docs/requirements.txt: pins mkdocs-material==9.7.6 so CI and local
  builds are reproducible.
• .github/workflows/docs.yml: builds the site and deploys to GitHub Pages
  using the modern Pages flow (configure-pages, upload-pages-artifact,
  deploy-pages) with pages: write / id-token: write permissions and
  concurrency on the pages group. Triggers on pushes to main that touch
  docs/**, mkdocs.yml, or the workflow, plus workflow_dispatch.
• User Guide link fix: four relative links that pointed outside docs_dir
  (../CONTRIBUTING.md x3, ../README.md x1) now use absolute GitHub URLs so
  mkdocs build --strict passes. The guide is otherwise unchanged. The built
  site/ directory is git-ignored.

Verification

mkdocs build --strict succeeds with no broken-link or nav warnings:

INFO    -  Cleaning site directory
INFO    -  Building documentation to directory: .../site
INFO    -  Documentation built in 0.37 seconds

All pages build (index, USER_GUIDE, performance, DEV_UV) and the search
index is generated. The guide's internal section anchors all resolve under the
default MkDocs slugify.

Action required to publish

GitHub Pages must be enabled once, manually: Settings -> Pages -> Source:
"GitHub Actions". Until then the deploy job cannot publish.

Open decisions (for the maintainer, not changed here)

1. Canonical manual vs. the wiki. The deep manual currently lives on the UT
   Austin / GitHub wiki. This site can grow into the canonical manual, or the
   wiki can remain the deep reference with this site as the in-repo essentials.
   Not decided here.
2. In-app Help link. Help -> Online resources still points the "user guide"
   at the wiki. Repointing it at this site (once published) is a separate
   decision and is intentionally left untouched.