8356975: Provide alternative way to generate preview API docs

[hns]

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 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.

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Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
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• Commit message must refer to an issue

Issues

• JDK-8356975: Provide alternative way to generate preview API docs (Enhancement - P4)
• JDK-8355933: Change section title for permanent APIs affected by preview features (Bug - P4)

Reviewing

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Update a local copy of the PR:
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[bridgekeeper]

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[openjdk]

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[openjdk]

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[hns]

/issue add JDK-8355933

[openjdk]

@hns
Adding additional issue to issue list: 8355933: Change section title for permanent APIs affected by preview features.

[mlbridge]

Webrevs

• 00: Full (e3f9a2b6)

[liach]

Should we add a check to assert there is exactly one occurrence of such a preview note tag?