Prune patchwork Sphinx config; add class inheritance diagrams

[levon003]

Addresses items 1, 2, 5, and 7 of #117 — the cleanup of the matplotlib/pydata template cruft in docs/conf.py.

Stacked on #118 (base is docs/fix-rst-warnings). GitHub will auto-retarget to main when #118 merges. Review/merge after #118.

Removed (all verified unused — make html SPHINXOPTS="-W --keep-going" is clean without them)

• sphinx.ext.linkcode + the 55-line linkcode_resolve() — verbatim matplotlib boilerplate that always returned None here (it guards on matplotlib/ paths). viewcode already provides the working [source] links. (item 1)
• The no-op docutils_conf dict — Sphinx never reads a module-level variable by that name. (item 2)
• numpydoc — the codebase is 100% Google-style (handled by napoleon), and its class-member tables were already disabled, so it did nothing. Dropping it also removes the numpydoc_show_class_members workaround. (item 5)
• The three matplotlib.sphinxext.* extensions and sphinx_design / sphinx_tags — no directive uses them. Their packages are removed from the docs dependency group too. (item 5)
• Now-unused conf.py imports (inspect, Path, parse_version, flexeval).

Net: conf.py loses ~87 lines.

Added

• Two inheritance_diagram diagrams on the API page (item 5 — "use it rather than remove it"): the polymorphic DataSource hierarchy and the peewee model hierarchy. These need Graphviz, so both CI workflows now apt-get install graphviz, and DEVELOPMENT.md documents the local prerequisite (brew install graphviz).
• DEVELOPMENT.md: notes that CI builds with -W (warnings = errors) and how to reproduce locally, plus a make linkcheck pointer (item 7 — documented, not automated, since it makes network requests).

Notes / left for you

• sphinxext.github (the local extension defining :ghissue:/:ghpull: roles) is also currently unused, but I kept it — it's a self-contained local utility (handy for release notes), not imported template cruft. Easy to drop if you'd rather.
• This is a docs/config-only change (nothing ships in the wheel), so no version bump — the 0.5.3 bump on Fix malformed-RST docstring warnings and enable -W in CI #118 covers the release.

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