Docusaurus migration: arbitrary block-level HTML attributes

[pdaoust]

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 <h2 id="run-a-happ">Run a hApp</h2> (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 <li class="style-blue">Style this in blue</li>
• > This blockquote is special {[data-special]} becomes <blockquote data-special>This blockquote is special</blockquote>