Update knowledge base to monorepo architecture

[mlewand]

📝 Provide a description of requested docs changes

Our knowledge base for sure will need to be adjusted once we're done with architecture change.

This is a subtask of #6479.

[mlewand]

During the v19.1.0 release we updated the release and testing instructions.

I wanted to already close this ticket, but I did a quick check and it seems that we still have few guides to update:

• bump-licence-year.md
• creating-repository.md

Might be more than that. So it make sense to keep it open.

[mlewand]

There's one more thing: https://github.com/cksource/ckeditor5-internal/blob/master/knowledge-base/preparing-the-testing-phase.md

Should:

1. cover creating (or merging master to) the release branch.
2. have changed git push origin master at the end to git push origin release