git: openjdk/amber-docs: Add new document on exhaustiveness, unconditionality and remainder

[duke]

Changeset: 0418b3f0
Author:    Gavin Bierman <gavin.bierman at oracle.com>
Date:      2023-05-23 16:17:21 +0000
URL:       https://git.openjdk.org/amber-docs/commit/0418b3f080ba4a85615d22db3445dafbcef12c61

Add new document on exhaustiveness, unconditionality and remainder

! site/_index.md
+ site/design-notes/patterns/exhaustiveness.md