Move the new mdoc(7) manual written by kristaps@, replacing the old one

Move the new mdoc(7) manual written by kristaps@, replacing the old one
which had much less information, such that the new one gets installed.
No change to the build system required, no text change in this commit.
Doing the big move early even though a few more improvements will follow.
The duplicate information in mdoc.samples(7) will be cleaned up post-release.
ok jmc@, and kristaps@ agreed with the plan, too