docs: prepare to include developer docs

[mmattel]

This PR PREPARES for the migration of the dev docs.

From the outside there are two points to mention:

• Nothing changes for existing links, all are page aliased
• We have a new admin module (book) where the former data is embedded
• The developer module (book) is prepared, "contentless" and currently removed from rendering

The changes include:

• Create the required structure
• Move the content formerly in the ROOT to the admin module
• Create page-aliases for all pages
• Update the nav structure
• Disable any rendering of the developer module by commenting the nav entry in ROOT
  • Prepare an index page for dev docs so we can easily enable/test
• Update of the repos site.yml file. Note that this only impacts local rendering