[foreign-memaccess+abi] RFR: 8277945: Use snippets in jdk.incubator.foreign API documentation

[Julia Boes]

This change replaces `<pre>{@code ...}</pre>` blocks with the new `@snippet` tag.

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Commit messages:
 - initial commit

Changes: https://git.openjdk.java.net/panama-foreign/pull/621/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=621&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277945
  Stats: 301 lines in 10 files changed: 0 ins; 0 del; 301 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/621.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/621/head:pull/621

PR: https://git.openjdk.java.net/panama-foreign/pull/621