[Docs] Add "How the documentation is organized" section

[dhmlau]

From https://youtu.be/8B-EWTQnzPI, @bajtos explained how the documentation is organized. This would be useful in helping users to navigate the documentation.

Discussed with @bajtos, it might be good to add a "How the documentation is organized" section at the end of the Overview page.

Acceptance criteria

• add a "How the documentation is organized" section at the end of the Overview page.
• update docs/site/README.md to avoid duplication of content (see [docs] Adopt four-quadrant documentation system #5718)

cc @bajtos