[harryswift01]

Summary

This PR moves the SIST documentation to the shared CCPBioSim Sphinx theme (ccpbiosim-sphinx-theme), so it matches the rest of the CCPBioSim repositories.

Changes

• Replace Furo and the unused docs packages (nbsphinx, sphinx_copybutton, sphinxcontrib-contentui and sphinxcontrib-details-directive) with ccpbiosim-sphinx-theme in docs/requirements.txt.
• Set the theme in conf.py and remove the Furo logo options and unused extensions.
• Add a banner, quick start tabs and section cards to the home page, with short labels in the navigation.
• Stop ignoring docs/source/_autosummary/, which is no longer generated.

Testing

• Built the docs in a fresh environment from docs/requirements.txt with make -C docs html SPHINXOPTS="-W --keep-going" with no warnings.
• Checked the home page in light and dark mode, the guide pages, the example downloads and the "Edit on GitHub" links.

Impact

• Docs changes only. The page content is the same apart from the home page layout.
• No API reference is added, as SIST is used through the sist command.
• The CI and Read the Docs setup already use docs/requirements.txt and docs/source, so they are unchanged.

Checklist

• Tests added or updated
• Documentation updated
• If dependencies in pyproject.toml changed, the lower bounds were reviewed deliberately

[jimboid]

Looks good