RFR: 8344301: Refine stylesheet for API docs [v2]

[Hannes Wallnöfer]

> This is an extensive update to the JavaDoc stylesheet that focuses on layout and typography, with the purpose of making API docs easier to read and use.
> 
> List of changes:
> 
>  - Slightly increase text and navigation fonts to improve readability and to better match the code font (which remains unchanged). 
>  - Fix font size and line height inconsistencies in many places.
>  - Limit the maximal width of page content to improve readability on large displays. If more horizontal space is available, page content is centered.
>  - Remove light gray section background in class and module pages to simplify and unclutter the layout. 
>  - Add a subtle gray background to code elements in normal text to set them apart.
>  - Highlight headings of member details when they are used as link targets.
>  - Reserve use of bold font in member summary tables to element names to make them easier to spot and reduce visual noise.
>  - Slightly reduce width of TOC sidebar and use ellipsis (...) if content does not fit (but long signatures continue to wrap).
>  - Make method signatures in details easier to read on small dispays by allowing them to wrap before parameters and throws sections.
>  - Make tabs on top of summary tables scroll instead of wrapping if they overflow the available horizontal space.
>  - Hide TOC sidebar and copy-to-clipboard buttons in addition to the top navigation bar when printing a page.
>  - Improve styling of additional HTML files in `doc-files` directories.
>  - Reduce rollover animations and remove smooth scrolling in order to improve accessibility.
>  - Add background to `<pre>` elements (will look great with [JDK-8346118](https://bugs.openjdk.org/browse/JDK-8346118)).
>  - Various cleanup.
> 
> There are too many non-trivial changes in the stylesheet to try explaining them here, but of course I'll be happy to discuss every change on request.
> 
> [Full JDK API docs are available for testing and comparison](https://cr.openjdk.org/~hannesw/8344301/api.05/index.html).

Hannes Wallnöfer has updated the pull request incrementally with two additional commits since the last revision:

 - Adjust copy button opacity
 - Changes:
    - reduce rollover transitions on headings and snippets
    - add background and padding to <pre> elements
    - slightly brighten snippets background

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23678/files
  - new: https://git.openjdk.org/jdk/pull/23678/files/1b6443e3..3deae2b5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23678&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23678&range=00-01

  Stats: 18 lines in 1 file changed: 8 ins; 3 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/23678.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23678/head:pull/23678

PR: https://git.openjdk.org/jdk/pull/23678