[foreign-memaccess+abi] RFR: Refresh FFM documents

[Maurizio Cimadamore]

This PR brings the FFM documents in sync with the finalized API in JDK 22. The most obvious changes are:

* in the foreign memory document, the section on structured access has been rectified, to take into account the extra offset parameter that is added to memory access var handles;
* in both documents, several calls to `allocateArray` and `allocateUtf8String` have been replaced with `allocateFrom`
* the section introducing the `Linker` in the ffi document has been revamped; it now talks more at length about the mapping between layouts and C types. The table with Linux/x64 mappings has been moved to a section in the appendix, and I have also added a mention of `Linker::canonicalLayouts`

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Commit messages:
 - Fix tab
 - Update Linker document
 - Refresh foreign memory API doc

Changes: https://git.openjdk.org/panama-foreign/pull/914/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=914&range=00
  Stats: 779 lines in 3 files changed: 39 ins; 705 del; 35 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/914.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/914/head:pull/914

PR: https://git.openjdk.org/panama-foreign/pull/914