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>

@fredlf @jamtur01 @ostezer
Maybe ..

> You can include a new document file (`.md`) by modifying the menu definition (`docs/mkdocs.yml`) file and adding it to the list of documents.