RFR: JDK-8298405: Support Markdown in the standard doclet [v3]

[Jonathan Gibbons]

> Support for Markdown comments in the standard doclet.
> 
> To enable Markdown in a comment, start the comment with `/**md` followed by whitespace.  The syntax is as defined for CommonMark.
> 
> The work is in 3 parts:
> 
> 1. Update the Compiler Tree API to support Markdown tree nodes, containing strings of (uninterpreted) Markdown source code.
> 2. Import commonmark-java into the `jdk.javadoc` module, to be able to convert Markdown strings to HTML.
> 3. Update the standard doclet, to leverage the preceding two parts, to translate Markdown in documentation comments to `Content` nodes.
> 
> There are new tests both for the low level work in the Compiler Tree API, and for the overall high-level work in the doclet.

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

 - Merge with upstream/master
 - Update copyright years
 - Rename FFFC variable
   Share Markdown parser and renderer in instance of MarkdownHandler
 - Move CommonMark to new internal module.
   Add legal header to imported CommonMark source files
   Always use Text nodes inside AttributeTree values
   Unwrap <p> from "simple" paragraphs
 - Always use Text nodes inside AttributeTree values
 - Update to CommonMark 0.21.
 - fix whitespace
 - JDK-8298405: Markdown support in the standard doclet

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Changes: https://git.openjdk.org/jdk/pull/11701/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11701&range=02
  Stats: 15635 lines in 145 files changed: 15343 ins; 97 del; 195 mod
  Patch: https://git.openjdk.org/jdk/pull/11701.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11701/head:pull/11701

PR: https://git.openjdk.org/jdk/pull/11701