docs: migrate to Zensical, refresh catalog

[tlambert03]

Moves the docs from MkDocs to Zensical and gives the colormap catalog a pretty big refresh.

Build and hosting:

• two-step build: python docs/_gen_catalog.py, then zensical build. .readthedocs.yaml is updated to match
• the docs workflow now only checks that the build passes, and no longer deploys to GitHub Pages (Pages should be turned off after merging, so we're only serving from RTD)
• the site is a lot lighter (about 208 MB → 50 MB). colormap pages are no longer listed in the sidebar... they're reached from the catalog, category and collection pages, and from search (with breadcrumbs)

Catalog:

• filter by name, collection, category and monotonic lightness, and the filters are kept in the URL so you can share them
• a page per collection (e.g. all of crameri), plus a collections index
• charts and test images are drawn in the browser rather than baked into each page:
  • color vision deficiency simulation (buttons marked with the missing cone, L / M / S)
  • sine ramp on continuous maps, plus a circular sine ramp (cyclic) and peaks (diverging)
  • discrete maps get one lightness point per color and no sine ramp
• the snippet tabs on each colormap page are cmap / matplotlib / plotly / napari / CSS / More... (with a short example for altair, bokeh, vispy, pygfx, pyqtgraph and Earth Engine). every example was run against current releases, except Earth Engine, which needs an account. the old napari example used napari.view_image, which is gone in napari 0.9... it now uses napari.imshow

New compare page:

• up to six colormaps side by side: colorbars, sine ramps and a lightness chart, with CVD simulation and a shareable URL
• lines get distinct dash patterns, and there's a toggle to color them with a fixed categorical set (petroff6) instead of the colormaps themselves
• you can pick, drop or paste your own image (it never leaves the browser) and see it in grayscale and through each colormap. two samples: a cat (mine), and elevation around Corsica (NOAA ETOPO 2022, CC0) with sea level at the middle, for diverging maps

Other stuff:

• dark theme by default
• pydantic docs and tests use v2 forms
• cmap._util.report() derivatives now use np.gradient, so they no longer start with a padded zero
• __all__ is reordered so the API page leads with Colormap and Color
• Kovesi links moved to Codeberg, and a typos pre-commit hook (fixed the typos it found)

RTD preview: https://cmap-docs--165.org.readthedocs.build/en/165/

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.73%. Comparing base (ea766cf) to head (3f186bf).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #165      +/-   ##
==========================================
+ Coverage   94.62%   95.73%   +1.11%     
==========================================
  Files         168      168              
  Lines        2232     2228       -4     
==========================================
+ Hits         2112     2133      +21     
+ Misses        120       95      -25     

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