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
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