RFR: 8266666: Implementation for snippets [v11]

[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:

  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.

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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/731a1a6b..0510287e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=10
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4795&range=09-10

  Stats: 2 lines in 2 files changed: 2 ins; 0 del; 0 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