Consistent header styles in docs

[philrz]

This is a pass through the docs to follow the new # / ## and related header conventions proposed in the intro text of #6569.

I intentionally did not touch the Tutorials since @mccanne mentioned he's looking at those at the moment. I also didn't touch Command since @mccanne mentioned he might be about to overhaul it. But if you'd like me to give those doc sets the same treatment in their current state, just say the word.

Since I had to go through pretty much every page to do this, I made another adjustment along the way of making sure there were blank lines in the Markdown between sections in the Operator/Function docs. Such blank lines were already present on most pages, so I just added them in the remainder

I also noticed a couple pages were alphabetically out of order in the left nav, so I adjusted those in the SUMMARY.md too.