RFR: 8318416: Superscript marks should use consistent font style

[Hannes Wallnöfer]

Please review a change to use consistent styles for the `PREVIEW` and `RESTRICTED` superscript labels in API docs. Previously, these labels inherited font style properties from their containing element, such as bold font in summary table links, or serif font in links contained in description text (see attached screenshots). 

With this change, superscript labels always use normal weight monospace font. Additionally, they now have a light gray background to make them easier to distinguish from surrounding text and give them a tag-like appearance.

Before / after screenshots:

<img width="773" alt="preview-summary-old" src="https://github.com/user-attachments/assets/fbb4e325-f706-4b54-88cd-34c207a2de25">

<img width="766" alt="preview-summary-new" src="https://github.com/user-attachments/assets/9cbe490d-0444-4c4f-a1f6-004754552166">

I also cleaned up the styles for `<sup>` elements in other contexts. Where the content of `<sup>` was previously [too big] or [too small] it now uses the browser default again. 

[too big]: https://docs.oracle.com/en/java/javase/23/docs/api/java.base/java/lang/Math.html#scalb(double,int)
[too small]: https://docs.oracle.com/en/java/javase/23/docs/api/java.base/java/lang/Math.html#expm1(double)

-------------

Commit messages:
 - Undo removal of line-height
 - 8318416: Superscript marks should use consistent font style

Changes: https://git.openjdk.org/jdk/pull/22393/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22393&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318416
  Stats: 156 lines in 10 files changed: 46 ins; 52 del; 58 mod
  Patch: https://git.openjdk.org/jdk/pull/22393.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22393/head:pull/22393

PR: https://git.openjdk.org/jdk/pull/22393