[doc] more fixes of warnings/errors in doc build - move 3.21.rst changelog description at it's proper place - include changelogs in the main index - few typos - add :noindex: for autoxxx directives not in api/ Related to #4832808

Upstream Author:

  %(author)s <%(author-email)s>

Copyright:

Copyright (c) %(year)s %(author)s.
%(author-web-site)s -- mailto:%(author-email)s