[foreign-memaccess+abi] RFR: Refresh FFM documents [v3]

[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`

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

  Address review comments

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/914/files
  - new: https://git.openjdk.org/panama-foreign/pull/914/files/eca4d75c..c951b5c3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=914&range=02
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=914&range=01-02

  Stats: 5 lines in 2 files changed: 0 ins; 1 del; 4 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