RFR: 8340818: Add a new jtreg test root to test the generated documentation [v4]

[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:

  - Add code block that was accidentally removed.
  - Simplify code

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21272/files
  - new: https://git.openjdk.org/jdk/pull/21272/files/d4dc02fe..f2e9f05b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21272&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21272&range=02-03

  Stats: 37 lines in 3 files changed: 31 ins; 1 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