RFR: 8340818: Add a new jtreg test root to test the generated documentation

[Nizar Benalla]

Please review this change that adds a new test root `docs` dedicated to testing the documentation, which has been a work in progress for a while. Tests for links, encoding, HTML, accessibility will be later added in following PRs. 

We also define a new make target `test-docs` meant for local use and depends on the docs.
This also adds the necessary configurations needed at Oracle.

This patch includes a test `TestDocs` which serves to show developers how they are meant to resolve the docs to test them, I want to include it temporarily until better tests are added later.

TIA

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Commit messages:
 - (C)
 - 8340818 - new test root

Changes: https://git.openjdk.org/jdk/pull/21272/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21272&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340818
  Stats: 211 lines in 10 files changed: 207 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/21272.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21272/head:pull/21272

PR: https://git.openjdk.org/jdk/pull/21272