RFR: 8266666: Implementation for snippets [v6]

[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 incrementally with two additional commits since the last revision:

 - 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>

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4795/files
  - new: https://git.openjdk.java.net/jdk/pull/4795/files/01afad63..0a47e9c2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=04-05

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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