RFR: 8352151: Fix display issues in javadoc-generated docs

[Hannes Wallnöfer]

Please review a patch to fix a number of display bugs in javadoc-generated documentation.

 -  Snippet copy button is transparent and can overlap with snippet content ([example][example-snippet])
 - After clicking on the second-to-last item in the sidebar, the last item is highlighted if the click triggers scrolling to the end of the page ([example][example-target])
 - No indentation on sidebar entries beyond the first two levels of hierarchy (only applies to [help page][example-indent])
 - (Minor) Add vertical margin to search page inputs so they look better when wrapped

[example-snippet]: https://download.java.net/java/early_access/jdk25/docs/api/java.base/java/lang/invoke/MethodHandle.html#asSpreader(int,java.lang.Class,int)
[example-target]: https://download.java.net/java/early_access/jdk25/docs/api/java.base/java/lang/Thread.Builder.html#start(java.lang.Runnable)
[example-indent]: https://download.java.net/java/early_access/jdk25/docs/api/help-doc.html

The sidebar highlighting script actually became simpler, which is great. The snippet copy button change is a bit more involved, because I had to change snippet layout to not overlay snippet content with the copy button, and make the button opaque. But the result is better than what we had before IMO. The other changes are simple one-liners.

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Commit messages:
 - 8352151: Fix display issues in javadoc-generated docs

Changes: https://git.openjdk.org/jdk/pull/24083/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24083&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352151
  Stats: 95 lines in 2 files changed: 19 ins; 45 del; 31 mod
  Patch: https://git.openjdk.org/jdk/pull/24083.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24083/head:pull/24083

PR: https://git.openjdk.org/jdk/pull/24083