RFR: JDK-8305958: Use links instead of buttons for auto-generated header links [v3]

[Hannes Wallnöfer]

> Please review a change to use plain HTML links instead of buttons to provide the URL to page section/heading anchors. Special care was taken to retain the accessibility features added in JDK-8299718, so section links can be reached and activated by keyboard and have an `aria-label` attribute. The generated documentation was tested on a range of desktop and mobile browsers (Firefox, Chrome on Mac OS and Linux, Safari on Mac OS and iOS, Chrome on Android) and can be viewed here:
> 
> https://cr.openjdk.org/~hannesw/8305958/api.00/index.html

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

  Move anchor-link sizing to stylesheet.css

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13831/files
  - new: https://git.openjdk.org/jdk/pull/13831/files/0be60c3c..efe0a741

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13831&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13831&range=01-02

  Stats: 5 lines in 2 files changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13831.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13831/head:pull/13831

PR: https://git.openjdk.org/jdk/pull/13831