Dan's Suggestions for the TDD tutorial

I'm making a few changes, happy to break this into a few more MRs or remove bits if people don't like them!

• Recommend that people create a run directory, and change the paths to act as though we're working from there.
• Replace the full URL to the EMPFlow.json in every link with a single [EMPFlow.json][pfcfg] (hopefully slightly more maintainable)
• Add a small hint about creating symlinks, in case people get tired of typing all the path names.
• In the hint about inspecting outputs after changing the jet calibration, mention h5ls and that if you remove all selection the number of jets should not change.
• shorten the paths to submit-single-btag: these days that script is added to the $PATH when you install the dumper, and it's executable, so no need to fully specify it.
• Add permalinks (#) to the sections. This is a global change to the ftag docs, not sure everyone wants that, but I find them helpful.

Again, happy to remove some of these, they are just suggestions!