gendoc: nest command headers under category headers Differential Revision: https://phab.mercurial-scm.org/D6329

minirst: support subsubsubsubsections (header level 5) with marker '''' Differential Revision: https://phab.mercurial-scm.org/D6328

gendoc: guarantee that all commands were processed The new logic renders the commands belonging to each category in turn. Commands with an unregistered category are at risk of getting skipped because their category is not in the list. By comparing the list of all commands to a log of processed commands, we can detect commands with unregistered categories and fail with an error message. Differential Revision: https://phab.mercurial-scm.org/D6327