gendoc: group commands by category in man page and HTML help Make Mercurial's man page and HTML help group commands by category, and present the categories in a helpful order. `hg help` already does this; this patch uses the same metadata. This patch uses the same header level for command categories and for commands. A subsequent patch will push the command headers down one level. Differential Revision: https://phab.mercurial-scm.org/D6326