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

[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

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

Commit messages:
 - JDK-8305958: Use links instead of buttons for auto-generated header links

Changes: https://git.openjdk.org/jdk/pull/13831/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13831&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305958
  Stats: 96 lines in 9 files changed: 54 ins; 32 del; 10 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