Docs: navigation tidy-up, content accuracy, and link fixes

[kilgor-trout-c22]

Summary

A cleanup pass over the documentation navigation and content. No new sections; focused on making the existing docs consistent, accurate against the current core (validated against humhub master, v1.18.3), and free of broken links.

Navigation

• Top navbar: converted About from a dropdown to a plain link, so every left item now behaves the same way (click to land, sidebar carries the sub-pages).
• Licenses: added a Licenses entry (https://www.humhub.com/licenses/) to the footer's More column (between Homepage and Marketplace).
• Sidebar:
  • Removed the License page from the Professional Edition sidebar and commented out the License link under About (both now covered by the footer Licenses link / the Commercial License link pointing to humhub.com/licenses/).
  • Added a commented-out HIDDEN block under Administration documenting the currently-orphaned pages (adminuser/*, user/*, admin/push-updates, admin/images) so they are easy to surface later.
  • Configuration group kept as a flat list (original order).

Content accuracy (validated against core v1.18.3)

• Corrected factual errors: cache/flush-all description, default upload folder (uploads/file), permission config path, async-tasks crontab path, enableServiceWorker default wording, missing fileNameValidationPattern description.
• Added missing console commands (module/info, module/register, ldap/show-user, test/db-connection) verified against the core command controllers.
• Added the missing Professional Edition module Limited User Admin (cross-checked the full PE list against the marketplace catalog).
• Documented the two guest-mode config options from feat: guest mode config options for hiding comments and expanding posts humhub#8190 as a commented-out block with a link to the (still open) PR, so they can be enabled once released.
• Fixed typos and normalized "Licence" to "License"; removed em dashes and other special characters.

Broken / stale links

• Fixed dead www.humhub.com/en/* links (professional-edition, cloud hosting, SaaS demo) and the "additional modules" link.
• Repointed the release-notes anchor to the correct Image Compression / Downscaling section.
• Pinned the Bootstrap 5 migration link to the v1.18.0 tag (stable regardless of branch movement).
• Routed a dead third-party link through the Wayback Machine.
• Refreshed the contribution page: removed the now-private GitHub project links and rewrote the "Contribute to the core development" section.

Verification

npm run build passes with no broken links or anchors. All external replacement URLs were checked and return 200.

Notes

• Kept a few footer links that currently 3xx-redirect (Marketplace, Twitter/X, Translation) as-is, since they still resolve.
• The Docker-first installation rework is intentionally not part of this PR; it lives on a separate branch.