[tox] Update doc testenv to account for documentation restructuring
* Build doc from `doc` directory.
* Use `sphinx-build` directly instead of `make` as this now works and allows
to build within .tox temporary environment (instead of doc/_build),
preferable for a "test" action.
Related to #5447161.
.. winclude:: search_sample_queries
You can as well type complex queries using the RQL_ query language,
used every where to build dynamic pages of this site.
You can use one of the following prefixes to specify which kind of search you
want:
* `rql` : RQL query
* `text` : full text search
.. _RQL: tut_rql