Move the documentation from MkDocs to Zensical

[g5t]

Material for MkDocs reaches end of life on 2026-11-05, after which it gets no further security fixes. Zensical is its successor from the same author, MIT and without the sponsorware model, and mkdocstrings works on it -- mkdocstrings' own site is built with Zensical and its author is on the team.

mkdocs-jupyter has no Zensical equivalent, and it was the only plugin without one. The notebooks are now rendered to Markdown with nbconvert, preserving their stored outputs exactly as execute: false did. All code, all outputs and all seven images survive; the 23-27% word-count drop on those pages is entirely mkdocs-jupyter's In[n]/Out[n] cell prompts. The rendered Markdown is a build artefact rather than a committed file -- docs/examples/build.py regenerates it, CI runs that before building, and .gitignore keeps it out -- so it cannot drift from the notebook it came from.

Two things surfaced by the move rather than caused by it:

• mccode_antlr.Flavor was cross-referenced from getting-started.md but documented nowhere, so the link had no target. MkDocs logged a warning and carried on; Zensical --strict aborts, with the file, line and column. Flavor is public and used in nearly every example, so it is now documented on the Assembler page and the reference resolves.

• docs/examples/display_geometry.ipynb was absent from the nav but still built and published at its URL. It is now in the nav, where its being published is at least deliberate.

The workflow moves from mkdocs gh-deploy --force -- which pushes to a gh-pages branch and needs contents:write -- to the Pages artifact flow, and now builds on pull requests too, so broken documentation fails before it merges rather than after.