RFR: 8338190: TOC vertical offsets not updated when documente size changes

[Hannes Wallnöfer]

Please review a simple change to switch to a [better suited API](https://developer.mozilla.org/en-US/docs/Web/API/ResizeObserver) to track changes in document size in `javadoc`-generated pages. In addition to changes in the size of the browser viewport, the new code handles changes coming from page content, such as showing different subsets of members in a member summary table.

This issue is labeled `noreg-hard` since we don't have a way to test resize behaviour in an automated way. I manually tested the change in Chrome, Firefox and Safari on various desktop and mobile platforms.

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Commit messages:
 - JDK-8338190: TOC vertical offsets not updated when documente size changes

Changes: https://git.openjdk.org/jdk/pull/20547/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20547&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338190
  Stats: 4 lines in 1 file changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20547.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20547/head:pull/20547

PR: https://git.openjdk.org/jdk/pull/20547