Minor doc-building fix-ups.

Put the sphinx-generated docs/Makefile under revision control like it
should have been from the start (requires some changes and fixes
to .gitignore).

Change project name from iperf to iperf3 in a few generated places in
the documentation pages, most notably the page headers.
Provide more useful links to get to content in a hurry.

While here some minor formatting tweaks.