RFR: JDK-8298405: Support Markdown in Documentation Comments [v32]

[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 44 commits:

 - fill in `visitRawText` in `CommentHelper.getTags` visitor
 - fixes for the "New API" page
 - change "standard" to "traditional" when referring to a comment
 - Merge remote-tracking branch 'upstream/master' into 8298405.doclet-markdown-v3
 - Merge remote-tracking branch 'upstream/master' into 8298405.doclet-markdown-v3
 - improve support for DocCommentParser.LineKind
 - Merge remote-tracking branch 'upstream/master' into 8298405.doclet-markdown-v3 # Please enter a commit message to explain why this merge is necessary, # especially if it merges an updated upstream into a topic branch. # # Lines starting with '#' will be ignored, and an empty message aborts # the
   commit.
 - update copyright year on test
 - refactor recent new test case in TestMarkdown.java
 - address review feedback
 - ... and 34 more: https://git.openjdk.org/jdk/compare/8765b176...2801c2e1

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Changes: https://git.openjdk.org/jdk/pull/16388/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16388&range=31
  Stats: 22094 lines in 197 files changed: 21411 ins; 286 del; 397 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