[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

Changes
-------

.. toctree::
    :maxdepth: 1

    3.21
    3.20
    3.19
    3.18
    3.17
    3.16
    3.15
    3.14