move roff papers from src to doc repository

[Ulrich Spörlein]

So, to get groff out of source we need to first remove the roff
docs/papers as we will no longer build and install them. As they still
contain conceivably useful information, I want to move them into the doc
repository.

This would be easy, if only we had one repository and not three. So this
loses history and is an ugly diff :/

The patch is here: http://people.freebsd.org/~uqs/roff_papers.diff

It's basically
- copy all sources over from src
- adjust Makefiles to remove SRCDIR
- connect to build

I also filed this as a PR under:
http://www.freebsd.org/cgi/query-pr.cgi?pr=178730

Cheers,
Uli