Retire mcpp which was superseeded by tradcpp.

ok drahn@
lgtm natano@
link cpp.1 to tradcpp.1 now that mcpp is gone

ok drahn@
lgtm natano@
Adjusts section 1 of release(8) to make it correct with Bitrig reality.
Initial bits for a complete rework over release(8).
Xr git(1) instead of cvs(1), while here, manually update .Dd, since
nothing will trigger $Mdocdate$ updating, it is a cvs keyword.
Introduces sources(7), a manual page to summarize information on Bitrig
system's source management code and kernel/userland building. This is
one more step towards a rework in the field of online information
resouces, specific for Bitrig's sources management, build and release
procedures.
Adding a few Xr to sources(7) from afterboot(8) and config(8)
Add newline before .Br.
Use .Lk for hyperlinks.
s/CD-ROM/signed release distribution/; from drahn@
Correct errata link.
Adding a section describing github repositories, wording improvements,
almost finished retrieving sources section, moving here tags/branches
explanation from release(8).
Adding more references to sources(7), moving tags/branches explanation
to sources(7), improving commands listing, some wording.
.Tn is supported only for compatibility
Shift introduction information on repositories to the top, thus
improving concepts flow.
sources.7 includes all information for building the kernel, remove some
duplication from release.8, also fix numbering.
Improve system build description
Moving details to sources.7, adding a Xr to sources and a note about the
history of this document on Bitrig.