refactor(docs): restructure documentation directory and framework

[juliapampus]

What this PR changes/adds

• Updates some basic markdown files (README, CONTRIBUTING, etc.): broken links, new URLs etc.
• Adds known_friends.md
• Moves extension, launcher and sample overview to their folder
• Fixes links that broke during project structure refactoring
• Restructures documentation directory to align with other EDC repos:

- docs
   - developer
      - decision-records
         - ...
         - README.md 
      - README.md
   - ...
   - templates

Why it does that

GitHub pages now have their own repo within this org.

Further notes

All links to the GitHub page are updated. Nevertheless, the current GitHub page will still be available and reference the new one. Docsify (reduced to the minimum with this PR) could be fully removed after some weeks/months.

Linked Issue(s)

Closes #1865
Closes #1844

Checklist

• added appropriate tests?
• performed checkstyle check locally?
• added/updated copyright headers?
• documented public classes/methods?
• added/updated relevant documentation?
• assigned appropriate label? (exclude from changelog with label no-changelog)
• formatted title correctly? (take a look at the CONTRIBUTING and styleguide for details)

[juliapampus]

New page available at: https://eclipse-dataspaceconnector.github.io/docs

[ndr-brt]

Fantastic! I think this is actually completing issue #1844