RFR: JDK-8298405: Implement JEP 467: Markdown Documentation Comments [v52]

[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 incrementally with three additional commits since the last revision:

 - 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.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16388/files
  - new: https://git.openjdk.org/jdk/pull/16388/files/03652d2f..477fb155

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16388&range=51
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16388&range=50-51

  Stats: 38 lines in 2 files changed: 26 ins; 0 del; 12 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