RFR: 8266666: Implementation for snippets [v8]

[Pavel Rappo]

> This PR implements JEP 413 "Code Snippets in Java API Documentation", which hasn't been yet proposed to target JDK 18. The PR starts as a squashed merge of the https://github.com/openjdk/jdk-sandbox/tree/jdk.javadoc/snippets branch.

Pavel Rappo has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 12 commits:

 - Merge branch 'master' into 8266666
 - Pass through FIXMEs and TODOs
   
   Downgrades FIXMEs that do not mark *feature issues* to TODOs, or removes those FIXMEs completely. For example, unlike Style hierarchy, Action hierarchy won't benefit from becoming sealed. So the respective FIXME is removed.
 - Update src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/toolkit/resources/stylesheet.css
   
   Co-authored-by: Hannes Wallnoefer <hannesw at gmail.com>
 - Update src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/toolkit/resources/stylesheet.css
   
   Co-authored-by: Hannes Wallnoefer <hannesw at gmail.com>
 - Change AnnotatedText<T> to StyledText
   
   Renames the class, removes its generic parameter, and changes the related terminology from "annotate" to "style".
 - Restructure ...toolkit.taglets.snippet.** packages
   
   This commit moves the contents of the jdk.javadoc.internal.doclets.toolkit.taglets.snippet.{action,parser,text} packages into the jdk.javadoc.internal.doclets.toolkit.taglets.snippet package.
 - Make CSS rules more specific
 - Improve comments
   
   This commit adds trivial commentary to some of the Java declarations. This commit also adds the "This is NOT part of any supported API..." disclaimer.
 - Update @since tags
 - Update copyright years
 - ... and 2 more: https://git.openjdk.java.net/jdk/compare/b7f75c0a...e1dbd98d

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

Changes: https://git.openjdk.java.net/jdk/pull/4795/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=07
  Stats: 4716 lines in 44 files changed: 4689 ins; 4 del; 23 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4795.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4795/head:pull/4795

PR: https://git.openjdk.java.net/jdk/pull/4795