RFR: 8340818: Add a new jtreg test root to test the generated documentation [v8]

[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

Nizar Benalla has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 11 additional commits since the last revision:

 - - add explicit env declaration to `run-test-prebuilt-docs` profile.
   
   - add TEST-DEPS solution
 - Merge branch 'master' into 8340818-jtreg-test-root
 - remove headers
 - - Add tidy as a dependency to `run-test-prebuilt-docs` profile
   - remove deprecated syntax in Main.gmk
   - add (C)
   - drop new test-docs target, left for future PR
   - drop list of keys in TEST.ROOT
 - Revert "Add tidy as a dependency to `run-test-prebuilt-docs` and `docs` profiles"
   
   This reverts commit f706a420b86196e56683eac6f37818edf97a7770.
 - Add tidy as a dependency to `run-test-prebuilt-docs` and `docs` profiles
 - - Add code block that was accidentally removed.
   - Simplify code
 - typos - remove a few mentions of doccheck
 - update after review comments
 - (C)
 - ... and 1 more: https://git.openjdk.org/jdk/compare/3e549b36...f5aeddd8

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21272/files
  - new: https://git.openjdk.org/jdk/pull/21272/files/2c61ba76..f5aeddd8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21272&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21272&range=06-07

  Stats: 9 lines in 5 files changed: 5 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