Mention the mkdocs.yml file for adding new documents

and move the complicated discussion about branches lower down,
hopefully most won't need to know

Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au>

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Merge pull request #8462 from SvenDowideit/document-how-to-add-a-new-document

Mention the mkdocs.yml file for adding new documents