Update top-level README to be more accessible, both for humans
and LLM's:
* Switch to Markdown syntax, rename file
* List the different documentation projects (qdocconf files),
with details of the content and how to build it.
* Defer for main building instructions to qt5.git, as a) a top-level
qt5.git build is the most reliable way to build the documentation,
and b) most advise here doesn't only apply to the *.qdocconf files
in qtdoc.git.
Change-Id: If903ec5283dcc6151e4628227637e422b98bc55e
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Kai Köhne