(doc) Extract sections of README_DEVELOPER

This pulls the various parts of README_DEVELOPER into individual files
and links to them from the docs/index.md. This should give us a place to
better organize these internal docs.
(doc) Remove remaining developer docs

The rest of the items in the developer docs (those not moved to the docs
directory) are of limited value. Instead of keeping these docs around
and having them fall out of date, they are just going to go away.
(doc) Explain some basics for profiling puppet

This doesn't make it all clear, but it should give a good starting point
for how you might go about profiling puppet.