RFR: 8266666: Implementation for snippets [v14]

[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 18 commits:

 - Merge branch 'master' into 8266666
   
   This merge is to promptly resolve a conflict in src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/toolkit/CommentUtils.java.
 - Clarify the attributes order comment
 - Fix import layout and typos
 - Add @summary to tests
   
   From https://openjdk.java.net/jtreg/faq.html:
   
   The @summary tag describes the condition that is checked by the test. It is especially useful for non-regression tests, which by definition don't have bug numbers, but even if there's a bug number it's helpful to include a summary. Note that a test summary is generally not the same thing as a Bugtraq synopsis, since the latter describes the bug rather than the condition that the bug violates.
   
   That said, the JBS synopsis for 8266666 suits those tests just fine.
 - Remove superfluous editor-fold
 - Remove JEP link from SnippetTree
 - 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>
 - ... and 8 more: https://git.openjdk.java.net/jdk/compare/d34f17c6...4c5e92b7

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

Changes: https://git.openjdk.java.net/jdk/pull/4795/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=13
  Stats: 4812 lines in 59 files changed: 4708 ins; 12 del; 92 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