Docusaurus migration: arbitrary block-level HTML attributes

On the current 11ty-based site, you can end a block with an attributes tag that takes the form of a CSS selector that gets converted to HTML attributes. Examples:

* `## Run a hApp {#run-a-happ}` becomes `

Run a hApp

` (note: headings should also get slugified IDs, but the slugifier chokes on words that start with a lower-case letter and have a capital letter in them somewhere)
* `* Style this in blue {.style-blue}` becomes `

Style this in blue

`
* `> This blockquote is special {[data-special]}` becomes `

This blockquote is special

`