[foreign-memaccess+abi] RFR: Beef up javadoc for base offset in var handles derived from layouts

[Maurizio Cimadamore]

This PR adds more javadoc to explain how base offsets in var handles obtained from layouts can be used.

First, a number of examples in the main `MemoryLayout` javadoc is rectified, as such examples did not use the additional base offset parameter (and were hence incorrect).

Second, the var handles part in the `MemorySegment` section on "accessing memory segment" has been rewritten to avoid duplication with the concepts exposed in `MemoryLayout`. Now we show how `MemorySegment::get` and `MemorySegment::getAtIndex` can be obtained using basic var handles and combinators. We mention that more complex var handles can be obtained using layout paths, but we now just reference the layout path section.

Last, I've added a new section on the `MemoryLayout` javadoc, with an example on how to deal with variable-length structs. This example showcases how var handles can be used in intricate situations, and combined with the new `scaleOffset` method.

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Commit messages:
 - Add example of layouts with variable-length structs
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/901/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=901&range=00
  Stats: 123 lines in 2 files changed: 78 ins; 17 del; 28 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/901.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/901/head:pull/901

PR: https://git.openjdk.org/panama-foreign/pull/901