Shipping documentation untarred

So the next step to documentation bliss is to get rid of the man.tar.gz and
postgres.tar.gz tarballs that are shipped inside the tarball. These are
historical artifacts from the era when building the documentation for release
required manual interference, and that era ended yesterday at the latest.

Here is how I would like to set this up:

* Man pages are built into doc/src/sgml/man1 and doc/src/sgml/man7. This is
already happening.

* HTML files are built into doc/src/sgml/html. On installation, we just copy
that directory.

* In doc/src/sgml/Makefile, put

distprep: man html

so that both documentation formats are built when the tarball is built.

An added twist is that derived files that are shipped in the release tarball
must be built in the source directory, not in the build directory (cf. gram.c
etc.). This is not a problem with a few options on the respective tools
(famous last words ...), but I just want to warn about it.

If people would find this behavior too weird for their personal development
workflow, we could add another target or other option to get the behavior you
want. Let me know.