Add an animated docs page explaining the section-tracing algorithm

[hdrake]

Stacked on top of MOM6-community#47 — base is topology-driven-neighbors, so this diff is docs only.

Adds docs/source/algorithm.md, a page explaining the section-tracing walk in
infer_grid_path. Today that algorithm is documented only by inline comments, and the
example notebooks show traced sections but never the mechanism. The centrepiece is an
animation stepping through every iteration of a two-segment, eight-step section on a small
synthetic grid, plus a static contact sheet of the same eight steps.

What the animation shows

Each iteration plays out in three beats — probe, admit, commit:

• probe — the four neighbours exactly as build_neighbor_maps returns them, with walls
  and the backtrack marked.
• admit — the admission circle, centred on the target with radius equal to the current
  corner's distance to it, so "admitted" is literally "strictly inside". Every admitted
  candidate arcs to both segment endpoints, on equal footing.
• commit — the winner in orange, with the two spherical angles whose sum is deviation
  shaded at the start and at the target. Both angles are drawn rather than just the
  target-side one, because their symmetry is exactly what makes the traced path independent
  of travel direction.

The grey background mesh is built from the neighbour maps rather than from index arithmetic,
so it is the graph the walk is actually allowed to move on.

The generator

docs/make_algorithm_animation.py produces both artifacts (~0.7 MB combined), committed
because Read the Docs never executes anything. It does not reimplement the walk: it
replays infer_grid_path to record per-step state for drawing, then asserts the replayed
path equals what grid_section returns. A change to the walk makes regeneration fail loudly
instead of letting the page drift out of sync.

Things to look at

• docs/environment.yml gains myst-parser and ffmpeg — existing docs
  environments need rebuilding.
• conf.py enables myst_parser; this is the repo's first hand-written .md page.
• docs/source/examples/ is now gitignored — conf.py regenerates and rmtrees it on every
  build, so it was untracked clutter.
• The two binary artifacts live under docs/source/_static/, which is not excluded from
  the sdist by pyproject.toml, so they ship to PyPI and stay in history.

Checks

• pytest — 76 passed, 0 skips.
• cd docs && make html SPHINXOPTS="-W" — clean, reproducing RTD's fail_on_warning: true.
  Video, contact sheet and all four notebook cross-links resolve in the built HTML.
• The page's runnable snippet was executed and its pasted output captured from the real run.
• All five example notebooks were re-executed cleanly against this branch. They are not
  included in this commit: no package code changes here, their sources came back
  byte-identical, and the only diff was nondeterministic PNG bytes.

---

Drafted with AI assistance (Claude Code); the commit carries a Co-Authored-By trailer, per
the project's AI usage policy.

🤖 Generated with Claude Code

[hdrake]

Looks great!