[foreign-memaccess+abi] RFR: Update docs

[Maurizio Cimadamore]

This patch updates the README of the foreign-memaccess+abi branch (which will soon become the new default branch for this repo).
It also updates the various docs to reflect the latest API changes

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Commit messages:
 - Reapply ffi doc changes
 - Tweak FFI doc
 - Initial push

Changes: https://git.openjdk.java.net/panama-foreign/pull/688/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=688&range=00
  Stats: 679 lines in 3 files changed: 674 ins; 2 del; 3 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/688.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/688/head:pull/688

PR: https://git.openjdk.java.net/panama-foreign/pull/688