RFR: 8340818: Add a new jtreg test root to test the generated documentation [v6]

[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 incrementally with two additional commits since the last revision:

 - - 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.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21272/files
  - new: https://git.openjdk.org/jdk/pull/21272/files/f706a420..69ef43e4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21272&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21272&range=04-05

  Stats: 150 lines in 9 files changed: 134 ins; 11 del; 5 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