[foreign-jextract] RFR: Refresh panama documents to reflect API changes

[Maurizio Cimadamore]

This patch tweaks the foreign memory access/linker documents to reflect the various changes in the API. Most notable changes are:

* The name of many static factories (e.g. layouts) has changed
* We dropped `MemorySegment::allocate` variants which do not accept a scope

I've also dropped the html version of said documents - since I believe the github rendering to be good enough (and, we refer to the github pages hosting these docs from other places anyway).

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Commit messages:
 - Fix minor spacing issue
 - * Fix issues with SegmentAllocator section
 - Update panama_memaccess.md
 - Update foreign documents

Changes: https://git.openjdk.java.net/panama-foreign/pull/535/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=535&range=00
  Stats: 1393 lines in 4 files changed: 13 ins; 1291 del; 89 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/535.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/535/head:pull/535

PR: https://git.openjdk.java.net/panama-foreign/pull/535