DOC modified sphinx's layout.hml

The previous sphinx layout relied on overriding a default template.
It meant we could not easily add links and other things in the header.
The new layout does not inherit from anything, hence makes it easier
to tune.
DOC the space between titles and the rest is now bigger
DOC improved spacing between functions, classes and the rest
Merge pull request #2186 from NelleV/doc_styling

DOC modified sphinx's layout.hml