[foreign-jextract] RFR: Update memory access and linker documents

[Maurizio Cimadamore]

This patch updates the two documents describing usage of the foreign memory access and foreign linker API.

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Commit messages:
 - Fix wrong order of links in README.md
 - Update docs

Changes: https://git.openjdk.java.net/panama-foreign/pull/475/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=475&range=00
  Stats: 2228 lines in 5 files changed: 1217 ins; 755 del; 256 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/475.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/475/head:pull/475

PR: https://git.openjdk.java.net/panama-foreign/pull/475