RFR: 8266666: Implementation for snippets

[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.

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Commit messages:
 - Update copyright years
 - Do not use snippets
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/4795/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266666
  Stats: 4603 lines in 44 files changed: 4575 ins; 4 del; 24 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