[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

Essayer le champ de recherche. Essayer des recherches comme "fiche
unmotachercher", ou encore "fiche wikiid index" ou "345".