Use HTML Living Standard sectioning

[jonathantneal]

The 1st commit:

1. Adds any normalized sections not already present from https://html.spec.whatwg.org/multipage/
2. Moves unsorted rules into their respective sections.

(which makes no changes to rendering, but is basically a setup for)

The 2nd commit:

1. Corrects the display for <summary> in all browsers, resolving Set <summary> elements to display:list-item #604.

@battaglr, would you have a moment to review?

[battaglr]

@jonathantneal

1. Using the new categories to define the code organization it's a nice improvement. The only drawback is that some elements may belong to more than one category, making us have to choose just one and maybe adding some confusion. But I think it's not a problem, just wanted to mention it just in case.

2. The code looks good, but there are some new interesting comments in Set <summary> elements to display:list-item #604 that are worth checking.

[Toddses]

#695