fix(markup): display MathML has no space below it (#39663) (#39668)

Backport #39663 by @nschloe

Since MathML is allowed in markup (#36352, #38034, #39337), a
display-style `<math display="block">` sits flush against the text below
it: it gets no margin of its own, and markup paragraphs have
`margin-top: 0`. Display math rendered by KaTeX gets 16px above and
below.

This adds `math[display="block"]` to the markup block elements that get
`margin-top: 0; margin-bottom: 16px`, like `p`, `pre` and `table`.

To test, view a Markdown file containing

```markdown
Text before.

<math display="block">
<mi>x</mi><mo>=</mo><mn>1</mn>
</math>

Text after.
```

Before, the equation has 16px above and 0px below; after, 16px on both
sides.

Before:

<img width="1544" height="238" alt="before"
src="https://github.com/user-attachments/assets/318238d9-018c-4945-974c-91bd1b282ff2"
/>

After:

<img width="1544" height="283" alt="after"
src="https://github.com/user-attachments/assets/9afde2be-b5ed-4624-a33f-4933e7fae79b"
/>

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Nico Schlömer <nschloe@users.noreply.github.com>
Co-authored-by: silverwind <me@silverwind.io>

web_src/css/markup/content.css

@@ -125,7 +125,8 @@
 .markup ol,
 .markup dl,
 .markup table,
-.markup pre {
+.markup pre,
+.markup math[display="block" i] {
   margin-top: 0;
   margin-bottom: 16px;
 }