RFR: 8298405: Implement JEP 467: Markdown Documentation Comments [v61]

[Jonathan Gibbons]

> Please review a patch to add support for Markdown syntax in documentation comments, as described in the associated JEP.
> 
> Notable features:
> 
> * support for `///` documentation comments in `JavaTokenizer`
> * new module `jdk.internal.md` -- a private copy of the `commonmark-java` library
> * updates to `DocCommentParser` to treat `///` comments as Markdown
> * updates to the standard doclet to render Markdown comments in HTML

Jonathan Gibbons has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 85 commits:

 - Merge remote-tracking branch 'upstream/master' into 8298405.doclet-markdown-v3
 - update commonmark-java from 0.21.0 to 0.22.0
 - Remove links to `jdk.javadoc` module from `java.compiler` module`
 - Suppress warnings building tests
 - Merge with upstream/master
 - address review feedback for updates to Elements and friends
 - address review feedback for updates to Elements and friends
 - add support for JDK-8329296: Update Elements for '///' documentation comments
 - add support for `--disable-line-doc-comments`
 - Merge branch '8298405.doclet-markdown-v3' of https://github.com/jonathan-gibbons/jdk into 8298405.doclet-markdown-v3
 - ... and 75 more: https://git.openjdk.org/jdk/compare/b96b38c2...20a384b6

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Changes: https://git.openjdk.org/jdk/pull/16388/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16388&range=60
  Stats: 26326 lines in 243 files changed: 25679 ins; 260 del; 387 mod
  Patch: https://git.openjdk.org/jdk/pull/16388.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16388/head:pull/16388

PR: https://git.openjdk.org/jdk/pull/16388