docs: add rules to make .html files

docs: add rules to make .html files

The documentation files are written in GFM (Girocco Flavored Markdown)
for which we conveniently have a formatter located in the markdown.git
submodule.

Add a suitable POSIX-compatible Makefile in the docs subdirectory
that knows how to process all *.txt files into *.html files using
the Markdown.pl script from the markdown.git submodule.

Add a "doc" rule to the top-level makefile to facilitate building
these "docs" and at the same time enhance the top-level "clean"
rule to actually do a clean.

The only things required to build the "docs" are:

 1) "perL" 5.8 or later is available in the $PATH somewhere
 2) the "markdown.git" submodule has been inited (i.e. checked out)
 3) a POSIX-compatible make is available

No other submodules need be checked out nor is a "special" version
of make required.

Just running:

    (cd docs && make)

will build the .html versions of all the docs provided the above
conditions have been met.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>