We previously introduced issues in our documentation due to not checking
for sphinx warnings [1], so let's enforce documentation validation.
This commit also fixes minor syntax issues caught by sphinx. Among
those, contribute.rst is now properly attached to ToC.
[1]: I48ab899077458d99eee6f643577d574106c1ca88
Change-Id: I9170883a20137c343f03d8b052399eacab4ae130