[foreign-memaccess+abi] RFR: Update docs [v3]

[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

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Improve both memory access and FFI documents

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Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/688/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/688/files/7de7a367..52689b87

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=688&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=688&range=01-02

  Stats: 72 lines in 2 files changed: 68 ins; 0 del; 4 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