RFR: Add guide section about array types

[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.

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Commit messages:
 - Add guide section about array types

Changes: https://git.openjdk.org/jextract/pull/236/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=236&range=00
  Stats: 61 lines in 1 file changed: 61 ins; 0 del; 0 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