[foreign-memaccess+abi] RFR: Add @sealedGraph tag to MemoryLayout

[Per Minborg]

This PR adds a `@sealedGraph` JavaDoc tag, opting in for rendering a graphical version of the sealed hierarchy of `MemoryLayout`

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Commit messages:
 - Add @sealedGraph tag to MemoryLayout

Changes: https://git.openjdk.org/panama-foreign/pull/735/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=735&range=00
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/735.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/735/head:pull/735

PR: https://git.openjdk.org/panama-foreign/pull/735