Adjust docs for monorepo

[mlewand]

Provide a description of the task

• scripts for building the docs (p3) - @pomek verified that both yarn run docs and integration build script for the entire CKEditor ecosystem will work out-of-the-box with no further changes required.
• dev articles in the docs (p4)
• markdown files in subpackages, like main README.md or api/docs/*.md like here (lines 23-30)

📃 Other details

This is a subtask of #6466.

[pomek]

scripts for building the docs

In CKEditor 5 repository works without any changes.

I needed to upgrade our documentation builder because now it does not need to bootstrap anything. Just a clone monorepo and link dependencies. And it's working.

[mlewand]

All right our public docs are done, except #6770 which has been extracted.