RFR: JDK-8298405: Implement JEP 467: Markdown Documentation Comments [v53]

[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 76 commits:

 - Merge branch '8298405.doclet-markdown-v3' of https://github.com/jonathan-gibbons/jdk into 8298405.doclet-markdown-v3
 - Merge pull request #3 from lahodaj/fix-positions-replacement
   
   Fixing positions when transforming javadoc text with replacement characters.
 - Merge branch '8298405.doclet-markdown-v3' into fix-positions-replacement
 - Fixing positions when transforming javadoc text with replacement characters.
 - Merge remote-tracking branch 'upstream/master' into 8298405.doclet-markdown-v3
 - update man page
 - Merge remote-tracking branch 'upstream/master' into 8298405.doclet-markdown-v3
 - Merge with upstream/master
 - Merge with upstream/master
 - improve first-sentence break in Markdown comments
 - ... and 66 more: https://git.openjdk.org/jdk/compare/3f2e849c...a8ea1c72

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Changes: https://git.openjdk.org/jdk/pull/16388/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16388&range=52
  Stats: 23635 lines in 209 files changed: 23012 ins; 254 del; 369 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