user-manual: move howto/make-dist.txt into user manual

user-manual: move howto/make-dist.txt into user manual

There seems to be a perception that the howto's are bit-rotting a
little.  The manual might be a more visible location for some of them,
and make-dist.txt seems like a good candidate to include as an example
in the manual.

For now, incorporate much of it verbatim.  Later we may want to update
the example a bit.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>