RFR: 8266666: Implementation for snippets [v7]

[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 one additional commit since the last revision:

  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.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4795/files
  - new: https://git.openjdk.java.net/jdk/pull/4795/files/0a47e9c2..22d1e07d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=05-06

  Stats: 11 lines in 5 files changed: 3 ins; 1 del; 7 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