RFR: 8356975: Provide alternative way to generate preview API docs [v2]

[Hannes Wallnöfer]

> Please review a change to allow marking API elements as preview features via a new hidden `--preview-feature-tag` option. In conjunction with the already existing hidden `--preview-note-tag` option this allows documenting elements as preview features without relying on JDK-internal types. 
> 
> Part of this change is a refactoring of how `SummaryListWriter` allows its subclasses to extend the content of summary pages. The old `addExtra*` hook methods were too unflexible as they always add extra content before other page content. I replaced them with protected `getIndexLinks` and `addSummaries` methods that allow overriding methods to add extra content before or after the default content returned by the super method.
> 
> The change also includes the fix for [JDK-8355933](https://bugs.openjdk.org/browse/JDK-8355933) to change the presentation and wording for elements with preview notes as requested by the Valhalla team. 
> 
> The output generated by the new test with preview features marked by tags [can be viewed here](https://cr.openjdk.org/~hannesw/8356975/out.00/api/package-summary.html).

Hannes Wallnöfer has updated the pull request incrementally with one additional commit since the last revision:

  Add warning for multiple preview notes

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26267/files
  - new: https://git.openjdk.org/jdk/pull/26267/files/e3f9a2b6..62aa0451

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26267&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26267&range=00-01

  Stats: 11 lines in 4 files changed: 11 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26267.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26267/head:pull/26267

PR: https://git.openjdk.org/jdk/pull/26267