[foreign-memaccess+abi] RFR: Update foreign memory doc

[Maurizio Cimadamore]

This patch updates the foreign memory document to reflect the latest API changes in Java 20.

I've streamlined the document, removed some obsolete sections (as well as sections that didn't seem to add much), and re-ordered the various sections so that the narrative is now easier to follow.

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Commit messages:
 - Fix bad footnote link
 - Update FFM API foreign memory doc

Changes: https://git.openjdk.org/panama-foreign/pull/758/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=758&range=00
  Stats: 221 lines in 1 file changed: 57 ins; 134 del; 30 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/758.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/758/head:pull/758

PR: https://git.openjdk.org/panama-foreign/pull/758