[doc] Start documenting the API Related to #4832808

[doc] Remove useless indexes Related to #4832808

[doc] Insert the dev docs in the toc Related to #4832808

[doc] Restructure the documentation * Create a new index file * Move the sphinx configuration files do the documentation root * Move book/README to dev/documenting.rst * Move book/mode_plan.py to tools/ * Move book/en/images to images * Move book/en/* to book/ * Move changelogs to changes/* * Adapt the Makefile * Add a title to the javascript api index Related to #4832808

[migration/3.21.0] performance and reliability fixes - use EXCEPT instead of NOT IN, seems to improve performance dramatically on postgresql - delete rows from cw_<etype> tables whose eid is unknown - deal with missing/dangling required inline relations (by deleting the subject entities) instead of asserting - fix/improve print statements - don't ask for confirmation before running pure select queries

[serverctl] fix source-sync command Closes #5491700

[server] missing import Fixes NameError in an unlikely error case.

[server/test] stop using email cube - make unittest_migractions use a separate datadir, and add a fake email cube there (we use it to test {add,drop}_cube, so need something that looks like a cube) - for the other test modules, just copy what we need from cubicweb-email's schema to our test schema

[web/views] Remove unused variable

[server/session] bring anonymous_connection back Fixes cubicweb-comment.