[foreign-memaccess+abi] RFR: Update docs [v2]

[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 five additional commits since the last revision:

 - Update doc/panama_ffi.md
   
   Co-authored-by: Jorn Vernee <JornVernee at users.noreply.github.com>
 - Update doc/panama_ffi.md
   
   Co-authored-by: Jorn Vernee <JornVernee at users.noreply.github.com>
 - Update doc/panama_ffi.md
   
   Co-authored-by: Jorn Vernee <JornVernee at users.noreply.github.com>
 - Update doc/panama_ffi.md
   
   Co-authored-by: Jorn Vernee <JornVernee at users.noreply.github.com>
 - Update doc/panama_ffi.md
   
   Co-authored-by: Jorn Vernee <JornVernee at users.noreply.github.com>

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

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/688/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/688/files/cf2adaa1..7de7a367

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=688&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=688&range=00-01

  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 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