Indent HEPML.tex, move Fast inference / deployment, and other fixes

[jmduarte]

This PR proposes two main changes, and two needed fixes

1. Automatically indent HEPML.tex using latexindent.pl pre-commit hook to make it more readable (also enforce it moving forward)
2. Move Fast inference / deployment up one level (so it's no longer a subcategory under Classification, but its own category alongside it). I believe this is a sensible change because these techniques are quite general and have been used for regression, reconstruction, etc.
3. I also fixed the missing second-tier headings (like Representations, Targets, etc.) in the README
4. For convenience, I also cherry-picked @j824h's Hbb fix from fix: Properly select } in \textbf argument #113

By the way, this came up because I'm trying to make a nomological net of ML in particle physics concepts and I thought it would be nice to re-use these categories. See here: https://github.com/jmduarte/Nomological_Net_ML_Particle_Physics

@bnachman @matthewfeickert can you review? Thank you!

[jmduarte]

I also just noticed @bnachman's issue: #60, which suggests to also move Representations into its own section as well. I'm happy to add that to this PR. (Should Targets be its own section too?)

[matthewfeickert]

Thanks for the PR! I'll review this tonight, but it is generally preferable to have PRs be as atomic as possible. This one combines multiple actions, so I might split this out and rebase it.