Docusaurus-style admonition blocks are not checked

[mcecode]

I know this syntax isn't in the CommonMark standard or even in GitHub-flavored Markdown, so this might be a no-go, especially since pulldown-cmark, the Markdown parser we use, only seems to support those two. However, I just want to document this here so others know that there might be issues when they're using this syntax in their Markdown, since I got tripped up a bit while editing our docs, which might also happen to others. I misspelled something inside an admonition block and thought that it was an actual word until I searched it. Interestingly enough, if you put the misspelled word outside of the block, it catches the mistake inside the block but not outside:

[hippietrail]

I believe the way Harper works is to divide the file up into spans of code/markup vs spans of comments/text, so it probably doesn't know what an "admonition block" is and treats it as part of the code/markup and doesn't lint it.