[book] fix docstrings to please sphinx example rst roles in docstring must be protected (in a literal).

[book] several fixes in the rst files - prevent duplicate markup definitions, - use existing Workflow markup instead of redefining one (in baseschema.rst) - include dataimport - fix non-existant directive autodocfunction - remove the empty 'embedding.rst' file

[book] activate the viewcode extension Automagically make link to class/modules/etc. it finds in the documents.

[book] new theme based on pyramid theme (closes #4291287)

Remove unused lgc.interface imports and leftovers

[repository] provide a .new_session entry point The current .connect only returns a `sessionid` which must be given to repo._get_session(...) to get the real session object. Related to #4151635.

[devtools/testlib] Use actual 'admin' user configuration values Previously ``db-user`` and ``db-password`` configuration values were used to fill ``admlogin`` and `admpassword`` attributes of CubicWebTC. The correct data for these should read from `[admin]` section. Closes #4065070.