Build the documentation for the website in the main repository

Add support (actually there was some existing but unused support here
already) for building the documentation parts of the website within the doc/
directory of the main repository, rather than within the www repository
(which had to checkout the main repository).  This makes it easy to build
the documentation and upload it to the webserver, as the webserver has
limited resources.

doc/Mmakefile:
    As above.
More support for building the documentation for the website.

doc/Mmakefile:
    Exit and report an error if the VERSION is "DEV".

    Add clean targets

    Make several other changes so that 'mmake webpage.tar.gz" works after
    "mmake distclean".