RFR: JDK-8298405: Support Markdown in Documentation Comments [v6]

[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 incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:

 - Merge with upstream/master
 - Merge remote-tracking branch 'upstream/master' into 8298405.doclet-markdown-v3
 - Address review comments
 - Fix whitespace
 - Improve handling of embedded inline taglets
 - Customize support for Markdown headings
 - JDK-8298405: Support Markdown in Documentation Comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16388/files
  - new: https://git.openjdk.org/jdk/pull/16388/files/39cdd2bb..50bb105d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16388&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16388&range=04-05

  Stats: 837199 lines in 5369 files changed: 205028 ins; 545552 del; 86619 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