[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
\.svn
^build$
^dist$
\.pyc$
\.pyo$
\.bak$
\.old$
\~$
\#.*?\#$
\.swp$
^doc/book/en/apidoc$
\.old$
syntax: regexp
.*/data.*/database/.*\.sqlite
.*/data.*/database/.*\.config
.*/data/database/tmpdb.*
.*/data/ldapdb/.*
^doc/html/
^doc/doctrees/
^doc/book/en/devweb/js_api/
^doc/_build
^doc/js_api/
data/pgdb/
data.*/pgdb.*