📚 Add GitHub repo link to docs sidebar

[ubmarco]

Summary

• Adds a Furo sidebar block linking to the GitHub repo (with repo slug and current version), mirroring the sphinx-codelinks docs.
• New template docs/source/_static/_templates/furo/side-github.html slotted into html_sidebars between scroll-end and the variant selector.
• New docs/source/_static/furo.css with .gh-source* styling; the icon is pinned to --color-foreground-primary so the GitHub mark stays monochrome (black on light, white on dark) instead of inheriting Furo's brand-purple link color. Body link colors are left at Furo defaults — narrower scope than codelinks, which overrides --color-link globally.

Test plan

• `sphinx-build -nW --keep-going -b html -T -c docs docs/source docs/_build/html` builds clean.
• Rendered `index.html` contains `` with the `useblocks/sphinx-mounts` label.
• Visually verify the icon is black in light mode and white in dark mode.