RFR: 8337111: Add system wide checks to the generated documentation

[Nizar Benalla]

Can I please get a review for this PR that adds 4 new html "Checkers" for the generated documentation.
More details are in the JBS issue

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Commit messages:
 - Convert parts of doccheck into tests

Changes: https://git.openjdk.org/jdk/pull/20711/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20711&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337111
  Stats: 1944 lines in 13 files changed: 1943 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20711.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20711/head:pull/20711

PR: https://git.openjdk.org/jdk/pull/20711