docs(site): organize guides by audience and simplify onboarding

[jouwdan]

Documentation mixed setup, member, operator and developer tasks, while the README repeated the entire documentation catalog. Organize the site around five sections: setting up, getting started, operating, using and developing Meith.

• Audit documentation against the code and correct installation, deployment, moderation, configuration, backup and recovery instructions.
• Add first steps and member guides, trim repetitive explanations, and simplify the README for newcomers and contributors.
• Group navigation by audience, retain existing published URLs, and generate the full documentation index in docs/README.md.
• Improve generated theme reference formatting while preserving property descriptions.
• Remove the default forum landing page’s “Community / Find your next conversation.” heading, retaining the mark-all-read action. The other bundled themes already omit this heading.

Validation:

• pnpm verify passed: 8,889 tests passed, 27 skipped.
• pnpm site:build, documentation link/index checks, comment checks and git diff --check passed.
• Checked built documentation navigation and rendered the forum homepage in fixture mode to confirm the heading is absent.

The full browser E2E suite and live deployment were not run.