[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
[doc] fix warnings/errors in doc build
- fix links to images
- fix a couple of typos
- re-add IDownloadableOneLineView doc
- rename documenting.rst back to .txt, it's intended as a doc of how to
write rst, not part of the rst doc
Related to #4832808