[foreign-memaccess+abi] RFR: 8304276: Add javadoc for custom arena

[Maurizio Cimadamore]

As the subject says, this PR adds a new section to the `Arena` javadoc, which describes how to build a "slicing arena". This example is useful to show how to build custom arenas.

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

Commit messages:
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/821/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=821&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304276
  Stats: 44 lines in 1 file changed: 44 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/821.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/821/head:pull/821

PR: https://git.openjdk.org/panama-foreign/pull/821