Doc: Include full LinuxDocTools code

BIRD uses hacked LinuxDocTools for building documentation, keeping some
parts locally and using remaining parts from system-installed one. This
setup breaks when LinuxDocTools makes some internal changes and is hard
to keep consistent.

Just include full LinuxDocTools code (both hacked and unmodified parts)
to avoid consistency issues. Note that we still need some binaries from
LinuxDocTools, so it still needs to be installed to build documentation.