gcc-8 documentation packages

[Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>]

Hello,

I've updated gcc-doc/gcc-doc-defaults packages to support new gcc-8
documentation generation. NMU Packages are uploaded to
mentors.debian.net
for review, git trees are put on salsa.debian.org/gcc-doc (-defaults).

-- 
With best wishes
Dmitry