docs: give the Sphinx docs the selfpatch.ai look

[loehub]

Pull Request

Summary

Restyles the docs site (selfpatch.github.io/ros2_medkit) to match selfpatch.ai and the new README. It keeps the Read the Docs theme, its layout and its navigation, so only the look changes.

• Fonts: Oxanium for headings, Inter for body text, IBM Plex Mono for code. They're self-hosted under docs/_static/fonts rather than loaded from Google Fonts, about 110 KB in total, each with its upstream licence file:
  • Inter and Oxanium: Latin subsets, which the OFL allows because neither reserves its font name.
  • IBM Plex Mono: IBM's own unmodified IBMPlexMono-Regular.woff2 from @ibm/plex-mono 2.5.0, because "Plex" is a reserved name. It also covers the box-drawing characters the directory trees use.
• Sidebar: graphite, with the selfpatch.ai wordmark above the project name. Section labels are sentence case instead of uppercase blue. The open section sits on a slightly raised panel and the current page is in acid.
• Content:
  • Links are forest green.
  • Inline code is ink on light grey instead of red.
  • Code blocks and notes are soft rounded panels; tips, warnings and danger keep tinted backgrounds.
  • Tables use hairline rows without zebra stripes or cell borders.
  • Cards are light grey panels without shadows.
  • sphinx-design tabs are ink instead of the default blue.
• Favicon: the selfpatch.ai icon.
• sphinx-needs tables: the existing DataTables fixes stay as they were, now in the same colours.
• Home page cards: the Quick Links grid gets an even 1.5rem gap both ways. It had none vertically, so the filled cards touched. Card grids now line up with the headings, text and code blocks around them.

Changes are limited to docs/_static/custom.css, two lines in docs/conf.py (html_logo, html_favicon), one :gutter: line in docs/index.rst and the static files above. The content width setting is unchanged.

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Issue

No linked issue. Suggested by the team in review chat.

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Type

• Bug fix
• New feature or tests
• Breaking change
• Documentation only

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Testing

• Built the docs locally with the pinned versions (Sphinx 8.2.3, sphinx-rtd-theme 3.1.0, sphinx-needs 6.3.0, sphinx-design 0.7.0) and served them.
• Checked the home page, Installation (tables, notes, inline code, code blocks), Getting Started, the Verification and Coverage pages (sphinx-needs, tabs, DataTables) and phone width.
• No new warnings. Doxygen and PlantUML weren't installed locally, so the API and diagram pages still showed their existing warnings. CI installs both and builds with -W.

To reproduce:

pip install -e docs/.[dev]
sphinx-build -b html docs docs/_build/html

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Checklist

• Breaking changes are clearly described (and announced in docs / changelog if needed)
• Tests were added or updated if needed
• Docs were updated if behavior or public API changed