Merge this site repo with m2e-core

[kwin]

The separation of repositories for the website from the actual code makes it hard/impossible to

1. find the relevant repo for documentation updates
2. update code along with documentation in the same PR

Therefore I propose to merge the site sources with the actual code in https://github.com/eclipse-m2e/m2e-core.

This would be similar to the setup from Tycho which contains the site sources in a dedicated directory in the main repo: https://github.com/eclipse-tycho/tycho/tree/main/src/site/