[Docs] Use relative urls when possible

See [this PR](https://github.com/beyond-all-reason/RecoilEngine/pull/2227), references in site pages should use site configuration data directly, or helpers to link to both repository pages (e.g. `github.com/bla/bar/asd.html -> repo_link("asd.html")`) and site pages (e.g. `something.github.io/site/bar.html -> site_link("site")`).

Jekyll and the jekyll template we use most likely offer both already, just figure a good one (or devise one if not good enough) and replace all hardcoded references on the site. The PR resolving this issue should serve as a reference to new page contributions as well.

Start by reviewing PR 2227 and the Jekyll configuration and template used by the site to identify existing URL helpers. Inventory hardcoded repository and site references, then replace them with configuration data or suitable helpers. Done means site links no longer use hardcoded URLs where relative helpers are available, and the resulting PR documents the pattern for future pages.

Written by the indexing model from the issue text.