Markdown inside `<details>` elements does not work

[weberc2]

This is due to an outstanding bug in the upstream markdown library.

<details>

<!-- ignore the backslashes; markdown in markdown is hard -->
\`\`\`yaml
foo
\`\`\`

</details>

Renders as

<details>

<!-- ignore the backslashes; markdown in markdown is hard -->
\`\`\`
foo
\`\`\`

</details>

Instead of

<details>

<pre><code>
foo
</code></pre>

</details>

[weberc2]

It seems the bug isn't going to be fixed in the upstream library, so Goldmark might work better--I think I've looked at it in the past and ruled it out as not being able to support some feature we required, but Goldmark does make a big deal about being extensible in its README, so it's probably worth a second look.

Reviewing the source code in markdown.go, it seems like these are the requirements for our markdown processing:

required extensions

• footnotes
• code fences
• tables
• autolink (detect and create links for URLs in posts)

goldmark builtin extensions

• footnotes
• code fences (part of commonmark i.e., not technically an extension)
• tables
• autolinks

modifications

• footnote links are converted to absolute so footnotes in snippets still link to the correct page.
  • presently this happens during rendering
• links to other markdown pages in this site are converted to links to their corresponding HTML files.
• it seems like all relative urls are converted to absolute urls (see patchURL())
• header deprecation (e.g., an h1 in markdown renders as an h2 in html to avoid conflicting with the toplevel h1)
  • presently we transform the ast right after parsing