[feat] Linter documentation in GitHub Pages.

[lukesneeringer]

This (incredibly large) PR adds a docs/ directory that we can serve with GitHub Pages.

It provides documentation for three linter rules; the remainder will come in follow-up PRs.

Note that as part of this, I intend to enforce (and soon lint for) some consistency on linter rules, including:

• One rule per file.
• The rule's filename must be derived from the rule name
  (converting - to _ but no other changes).

This is to make things like "View Implmenetation" easy.

[lukesneeringer]

The front page looks nice, but needs installation instructions (which can also come in a followup, preferably by not-me):

[lukesneeringer]

The base page for rule documentation will eventually have tiles for the groups of rules. Right now only core has any rules implemented, so there is only one. This is fine.

[lukesneeringer]

The rule listing has an expandable "zippy" for each AIP:

[lukesneeringer]

Each rule gets a page, like in the internal linter. There is a quick button on the right that takes you to the implementation code in GitHub. Incorrect and correct code get the red or green line on the side, which interacts better with syntax highlighting than does changing the background color:

[lukesneeringer]

The page overall looks good on mobile, although protos that are too wide will cause horizontal scrolling, as seen in the comments. That's probably just life.

[lukesneeringer]

Print mode looks good too, although I need to figure out what is up with the big blank space on the top. I bet that is broken on the AIP site also.

Note that the incorrect and correct protos now start with // Incorrect. and // Correct. (That's actually the magic that causes the green and red lines in the screen mode; I intentionally throw away those comments on the screen but preserve them in print.

[lukesneeringer]

This is not perfect, but it is definitely good enough. I want to merge it and then make incremental improvements. This will also unblock getting documentation written for other rules.

[mingzhi]

The pages look wonderful!!! And thank you so much for walkthrough on different pages! I quickly approved this PR without deep review. And as you said, we can make more incremental changes after this PR.