RFR: JDK-8308659: Use CSS scroll-margin instead of flexbox layout in API documentation

[Hannes Wallnöfer]

A few years ago we switched to [Flexbox Layout](https://developer.mozilla.org/en-US/docs/Glossary/Flexbox) to implement the sticky header in the API docs generated by `javadoc`. This solved the problem of anchor link targets [not being positioned correctly](https://bugs.openjdk.org/browse/JDK-8223378), but it also introduced a few new ones:

 - It required a workaround to get browser history to work (JDK-8249133, JDK-8250779, 8286832)
 - It changed certain aspects of scrolling behavior in the browser (JDK-8301080)
 - It changed the way some CSS properties are interpreted (JDK-8315800)

The reason for most of these problems is that the layout paradigm used by Flexbox is very different from traditional layout of HTML pages. The `scroll-margin-*` CSS properties introduced by the [CSS Scroll Snap Module](https://www.w3.org/TR/css-scroll-snap-1/) provide a simpler and less intrusive way to implement link target offsets in combination with sticky elements implemented using [`position: sticky`](https://developer.mozilla.org/en-US/docs/Web/CSS/position). However, although it is implemented [in all supported browsers](https://caniuse.com/?search=scroll-margin), it comes with its own challanges and quirks, which are explained below.

The first problem to overcome was that `position: sticky` is more fragile on mobile browsers than Flexbox. If some part of the page content overflows the allotted horizontal space, the whole page can be zoomed out to view the whole content. This causes the header to become unsticky and the link target offsets to become erroneous. It was therefore necessary to make sure page content never overflows its allotted horizontal space, either by adding line break opportunities, or by making all or part of the page horizontally scrollable when its content overflows. Line break opportunities are difficult to add especially in  preformatted code, so I opted for making the parts of the page most likely containing long lines of code scrollable. 

The next problem was that enabling horizontal scrolling on an element or one of its containing elements breaks the `scroll-margin-top` property in Chrome due to a browser quirk (both desktop and mobile versions). This means that the scrolling must occur in a child element rather than the sections or other elements serving as link targets. 

When enabling horizontal scrolling on the contents of sections containing user-provided content, another problem is that it disables [Margin Collapse](https://www.joshwcomeau.com/css/rules-of-margin-collapse/) which regulates margins between adjacent and contained boxes. Since these sections can contain a mix of HTML containers, it is almost impossible to preserve the margins between elements while making them scrollable. 

The solution to these problems was to add a new `<div class="horizontal-scroll">` to the sections containing user-provided content. Adding a single scrollable element per section allowed the margins to be controlled in a relatively safe way, although some CSS is introduced to limit the margin of certain trailing block elements in certain sections. 

The `scroll-margin-top` property is set to the normal height of the sticky header in `stylesheet.css`, with a line added to `search.js` to set the property dynamically to the actual header height to accommodate for the pre-release notification banner. In a previous version of this branch, the pre-release banner was excluded from the sticky header, but it was made sticky again in the final version to make the output more comparable. 

The generated documentation should render the same on supported browsers pixel by pixel. Before/after snapshots of the JDK docs are available here:

Before: https://cr.openjdk.org/~hannesw/8308659/api.00/
After: https://cr.openjdk.org/~hannesw/8308659/api.01/

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

Commit messages:
 - Rename local variable
 - Make enum constant detail scrollable
 - Avoid nested rules
 - Fix summary page table headers
 - More CSS changes
 - Further cleanup
 - Refine style adaptations
 - Update copyright header dates
 - JDK-8308659: Use CSS scroll-margin instead of flexbox layout in API documentation

Changes: https://git.openjdk.org/jdk/pull/15969/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15969&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308659
  Stats: 296 lines in 38 files changed: 108 ins; 75 del; 113 mod
  Patch: https://git.openjdk.org/jdk/pull/15969.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15969/head:pull/15969

PR: https://git.openjdk.org/jdk/pull/15969