[IGNORE] Migration proof of concept

[parlough]

This isn't done and there are many TODOs, documented both here and elsewhere, but it showcases the site at least building with 11ty.

Some initial details:

• Simpler aside/admonition syntax, also allows for custom titles.
• Staging, link checking, testing, etc on GitHub went from 7+ minutes -> ≈1-2 minutes!
• Code highlighting and custom styles ([! !], {:.good}, etc) are not yet functional.
• There are a bunch of minor issues marked with TODOs throughout the site.
• Files beginning with _ are still rendered, this likely needs to be disabled.

Staged: https://dart-dev--pr5284-feature-migrate-to-11ty-fpccvu5v.web.app/

[khanhnwin]

This looks great @parlough!!

[MaryaBelanger]

The site looks great! Site alerts and code blocks seem more distinct and styled better too (even without {:.good} / {:.bad} yet!)

Edit: I just saw one of your "Non-goals" : Change site output in any significant way.
So, uhh... nevermind, it looks exactly the same...

[parlough]

Okay code blocks and site alerts will be two exceptions and will change, but those changes are more side effects than goals :)

They're two of the last remaining steps though, so expect them to change further.

[dart-github-bot]

Visit the preview URL for this PR (updated for commit 2c3aff1):

https://dart-dev--pr5284-feature-migrate-to-11ty-fpccvu5v.web.app

[parlough]

Superseded by #5483