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