Drop the archived readthedocs-sphinx-ext, which breaks the docs build on Sphinx 8.2+

[dylanpulver]

The Read the Docs build is failing on current PRs, and it will fail on every new one. readthedocs-sphinx-ext runs this on each page:

if inject_css and theme_css not in app.builder.css_files:

css_files was removed from Sphinx's HTML builders, so the attribute access raises before the sphinx.version_info < (1, 8) guard below it can help. Since doc pins readthedocs-sphinx-ext>=2.2 but leaves sphinx unpinned, each build resolves a newer Sphinx and dies:

Extension error (readthedocs_ext.readthedocs)!
  if inject_css and theme_css not in app.builder.css_files:
AttributeError: 'StandaloneHTMLBuilder' object has no attribute 'css_files'

That is build 34785300 verbatim, and it reproduces locally with .[doc] and doc/conf.py unchanged.

Bisected against the extension at 2.2.5: Sphinx 8.1.3 builds, 8.2.3 and everything after fails.

Why remove it rather than pin Sphinx. The extension is archived upstream, so it will not be fixed and a pin would hold CLIMADA on Sphinx 8.1 indefinitely. Its job is injecting sphinx_rtd_theme or badge_only CSS plus the old doc-embed JS; CLIMADA uses sphinx-book-theme, so that CSS was never wanted here, and Read the Docs Addons injects the flyout and search itself now. Nothing under doc/ references readthedocs_build, readthedocs_ext or those assets, and conf.py's own readthedocs config value is declared in its setup() and is unaffected.

With the extension dropped, the same command Read the Docs runs succeeds:

build succeeded, 52 warnings.

No warning mentions Read the Docs or CSS, and the warning count is what the build already carried.

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