RFR: 8340818: Add a new jtreg test root to test the generated documentation [v11]

[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 one additional commit since the last revision:

  use existing DOCS_JDK_IMAGE_DIR to be consistent.
  
  no ifneq check before assigning the env variable because  DOCS_JDK_IMAGE_DIR is set even docs do not exist. It will be up to the test to check if the files are there

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21272/files
  - new: https://git.openjdk.org/jdk/pull/21272/files/25668d0f..3cb1c367

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21272&range=10
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21272&range=09-10

  Stats: 8 lines in 3 files changed: 0 ins; 5 del; 3 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