[clue]

This changeset updates mkdocs-material from 8.1.3 (2021) to 9.8.0. Version 9 made the edit button, the previous/next footer and the code copy button opt-in, so this enables them again. Material for MkDocs reaches its end of life on May 5, 2027, with Zensical as its maintained successor.

I've compared every page before and after: search shows richer previews, tabbed code blocks are slightly more compact and the sidebar section headings are a bit lighter, otherwise the documentation looks the same.

Builds on top of #100 and #271

[Copilot]

🟢 Approval recommended

No active review issues were identified for the documentation theme upgrade and explicit opt-in feature configuration.

No open issues 🎉

What changed in this PR

Updates the Docker Compose documentation setup to Material for MkDocs 9.8.0 while preserving features that became opt-in in version 9.

Changes:

• Upgrades the documentation container image from 8.1.3 to 9.8.0.
• Explicitly enables edit links, code-copy buttons, and previous/next footer navigation.

File	Description
docs/​mkdocs.yml	Enables the three opt-in theme features and removes trailing whitespace.
docs/​compose.yaml	Updates the Material for MkDocs image to 9.8.0.

_{Review effort: Balanced}