Restore the Read the Docs build and refresh the docs

[gouwens]

The hosted docs at https://ccf-streamlines.readthedocs.io are not rebuilding.
.readthedocs.yaml has asked for build.os: ubuntu-20.04 since October 2022,
and Read the Docs removed that image on 2026-06-01,
so the build now fails before Sphinx starts. The published site is serving a
snapshot old enough to predate the Glossary page.

Build

• ubuntu-24.04 / Python 3.12. The docs build does not install the package
  (conf.py mocks the third-party imports and regex-scrapes the version out of
  pyproject.toml), so this interpreter is independent of the requires-python
  floor in pyproject.toml.
• docs/requirements.txt moves to Sphinx 9.1.0 / pydata-sphinx-theme 0.21.0.
  The old pins could not simply be carried forward: Sphinx 5.2.3 imports
  imghdr, which Python 3.13 removed.
• sphinx.fail_on_warning: true, now that the build is warning-free. Dropping
  html_static_path (the tree keeps no _static directory) removed the only
  warning.

Content

Found by checking each page against the current code and by running
sphinx-build -b linkcheck:

• The first Isocortex2dProjector example was missing a comma after
  hemisphere="both", so copying it out of the guide raised a SyntaxError.
  The coordinate projector example asked for flatmap_buttefly.h5.
• The CCF white paper link redirected to a forum category page. It now points at
  the 2017 technical white paper, verified to contain the passage the guide
  quotes and the Figure 4 that ccf_whitepaper_figure_4.png shows.
• New Installation section: the docs never said pip install ccf-streamlines,
  that imports are by submodule, or that the reference files are downloaded
  separately.
• reference/processing.rst was titled ccf_streamlines.projection.
• Allen download links upgraded to https (they permanently redirect there), plus
  three prose typos.

linkcheck is clean apart from cell.com, which answers every non-browser client
with a 403; that is recorded in linkcheck_ignore with the reason.

Checks

• sphinx-build -b html -W --keep-going on Python 3.12 with the new pins:
  build succeeded, no warnings.
• uv run pytest: 366 passed, 36 skipped.

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