RFR: 8344041: Re-enable external specs page

[Hannes Wallnöfer]

Please review a change to enable the External Specifications page in API docs. The page was disabled because of missing `@spec` tags in OpenJDK sources. This is a straightforward undo of of #13127 / [JDK-8304689], which means the hidden ad-hoc `--no-external-specs-page` option is also removed.

[JDK-8304689]: https://bugs.openjdk.org/browse/JDK-8304689

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Commit messages:
 - JDK-8344041: Re-enable external specs page

Changes: https://git.openjdk.org/jdk/pull/22240/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22240&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344041
  Stats: 47 lines in 4 files changed: 0 ins; 42 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/22240.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22240/head:pull/22240

PR: https://git.openjdk.org/jdk/pull/22240