doc/templates/extheader.txt

doc: generate separate commands/topics/extension pages

This change modifies gendoc.py and Makefile so that individual pages for
commands, help topics, and extensions can be generated. A new index page is
also generated with links to all these pages. This makes it easier to look up
and search the help text of a given command or topic, instead of having to
deal with the giant hg.1 "all-in-one" page.

Since the list of individual pages varies based on the source code, we generate
a dynamic Makefile that contains this list of files as individual targets.
This gives us fine-grained control over output files. However, it greatly
increases the time spent generating all help pages. It's recommended to run
make with -j to make use of multi-core archs.

Individual man pages are produced in doc/man, and HTML ones are in doc/html

.. _ext-%(extname)s:

%(exttitle)s

.. contents::
   :backlinks: top
   :class: htmlonly
   :depth: 2