RFR: Add guide section about array types [v2]

[Jorn Vernee]

> The current guide is missing a section discussing the special handling of array types, for which jextract generates a `$dimensions()` accessor, and indexed getters and setters.
> 
> This PR adds a section to the guide discussing this. I've put it at the end of the "Using The Code Generated By Jextract" section. I've put it before the section on nested types though, since it's a little more important I think.

Jorn Vernee has updated the pull request incrementally with three additional commits since the last revision:

 - review comments
 - Update doc/GUIDE.md
   
   Co-authored-by: Maurizio Cimadamore <54672762+mcimadamore at users.noreply.github.com>
 - Update doc/GUIDE.md
   
   Co-authored-by: Maurizio Cimadamore <54672762+mcimadamore at users.noreply.github.com>

-------------

Changes:
  - all: https://git.openjdk.org/jextract/pull/236/files
  - new: https://git.openjdk.org/jextract/pull/236/files/d8dc0b6d..635d95f6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=236&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=236&range=00-01

  Stats: 22 lines in 1 file changed: 7 ins; 7 del; 8 mod
  Patch: https://git.openjdk.org/jextract/pull/236.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/236/head:pull/236

PR: https://git.openjdk.org/jextract/pull/236