Add Sphinx documentation site with TikZ figures and CI deploy workflow

[Kiguli]

Summary

Adds a new docs/ Sphinx site (Furo theme, sphinx_design cards,
MyST, sphinx-autoapi, sphinxcontrib-mermaid) covering everything
BehaVerify offers, plus a GitHub Actions workflow that builds and
publishes it to GitHub Pages on every push to main.

What lands

• Landing page with a TikZ-rendered pipeline diagram showing the
  five-stage flow: inputs (.tree / .onnx / .smv) →
  core pipeline (TextX parser, model builder, internal IR) →
  code generators (nuxmv / python / cpp / haskell / latex /
  monitor) → artefacts → three verification strategies.
• Getting started: installation (incl. nuXmv + ONNX), quickstart
  on examples/DrunkenDrone/, first-model tutorial.
• User guide: DSL reference, component taxonomy with a backend
  coverage matrix, generation modes, specification logics (INVAR /
  LTL / CTL + contingency monitors), neural-network leaves,
  encodings, and verification strategies.
• Theory (new dedicated section, grounded in the four BehaVerify
  papers): behavior-tree formalism with Colledanchise-style oval
  conditions / rectangle actions, specification logics, BT → SMV
  translation (fast-forwarding + variable staging), model checking
  with nuXmv (BDDs / BMC / IC3), end-to-end soundness argument with a
  two-row serpentine chain diagram, neuro-symbolic encoding, stateful
  BTs, contingency monitors.
• Examples: DrunkenDrone, Collatz, ACAS-Xu, Simple Robot with NN,
  and Grid-World (the 2055-node scalability benchmark).
• Developer: architecture, adding-a-mode, adding-a-check, testing,
  contributing.
• Reference: CLI, file formats, glossary, publications.

Every tree diagram and the pipeline / encoding / soundness figures are
authored in TikZ under docs/_static/tikz/ and pre-rendered to SVG +
PNG, so the Sphinx build does not require a LaTeX toolchain.
Regenerate with make -C docs tikz locally.

CI / deployment

.github/workflows/docs.yml builds the site on:

• every push to main or master that touches docs/**,
  src/behaverify/**, or the workflow itself;
• every PR targeting main / master (as a preview — uploads the
  HTML as a downloadable artifact, no deploy);
• manual trigger from the Actions tab via workflow_dispatch,
  which lets a maintainer rebuild and redeploy without a new commit.

Deployment uses the official GitHub Pages actions
(configure-pages@v5, upload-pages-artifact@v3,
deploy-pages@v4) — no third-party tokens.

Maintainer steps before merging

1. Go to https://github.com/verivital/behaverify/settings/pages and
   set Source = GitHub Actions.
2. (Optional) Add a github-pages environment under
   `Settings → Environments` restricting deployment to the `main`
   branch.
3. Merge this PR — the first build will fire and publish to
   `https://verivital.github.io/behaverify/\`.

Full step-by-step guide (including the manual rebuild path and TikZ
re-render instructions) is in `docs/DEPLOYMENT.md`.

Test plan

• `sphinx-build -b html -W --keep-going -n docs docs/_build/html` passes locally (0 warnings).
• `make -C docs tikz` regenerates all 12 figures cleanly.
• All existing CI (`ci.yml`, `pylint.yml`, `publish.yml`) untouched.
• After merge: verify the `Documentation` workflow's first run publishes to Pages.
• After merge: `workflow_dispatch` from the Actions tab triggers a manual rebuild.