Make MAINTAINERS.md more nuanced
Make MAINTAINERS.md more nuanced

This is a first step to get away from a single person carrying all the
weight for the whole, quite involved repository.

I would envision that we define more focus areas with more maintainers
later. This change should already help by directing project-specific
tasks to the respective project maintainers. I also added Julius for
the "technical" part of the site generation.

Signed-off-by: beorn7 <91e32ae1d26bd312b370cf8378a5a83accca90a0@grafana.com>

prometheus/docs,prometheus/docs,prometheus/docs