ci(docs): add MkDocs site + GitHub Pages deploy

[adworzynski]

Stands up a documentation site (MkDocs + Material) built from the existing docs/ tree and deploys
it to GitHub Pages — part of the go-public work.

• mkdocs.yml — Material theme, light/dark toggle, explicit nav over principles / architecture /
  reference / guides / decisions. Homepage reuses docs/README.md (MkDocs maps README.md→index),
  now with a short thesis lead.
• .github/workflows/docs.yml — builds with mkdocs build and deploys via
  upload-pages-artifact + deploy-pages. Trigger: push to main (paths-filtered to docs/**
  • mkdocs.yml + the workflow) plus workflow_dispatch, so docs publish when a change lands — not
    gated on releases.
• pyproject.toml — new docs optional-dependency group (mkdocs, mkdocs-material), version-capped
  below the announced breaking mkdocs 2.0 / mkdocs-material 10.
• .gitignore — ignore site/ / _site/ build output.

Note: the deploy job only succeeds once Pages is enabled (source = GitHub Actions) and the
repo is public — both happen in the go-public flip step. Until then the workflow is wired but the
deploy is expected to no-op/fail. Build validated locally (mkdocs build, exit 0; only the
known ../proposals/ cross-links warn, harmless in non-strict mode).

Repo infra / docs-only — no src/ change.