Squashed 'docs/_esnet/' content from commit c02bf57

git-subtree-dir: docs/_esnet
git-subtree-split: c02bf57d43dbfcd7c727cdd17c54f0a480279f9a
Merge commit 'a3356c98134c9e2995207a00c40276587de05bc2' as 'docs/_esnet'
First pass at new iperf3 documentation, implemented in reStructuredText.

Building and deploying the documentation requires some infrastructure
that's not part of this tree.