chore: add document about compliant vs non-compliant apps

[mcdurdin]

References should be made from /knowledge-base/kb0111, /developer/language/reference/_nul, and probably quite a few other places as well. Target audience should be keyboard authors and advanced users (and thus developers/integrators by inference) -- most people won't need to know or understand the difference.

Wiki references:

• https://github.com/keymanapp/keyman/wiki/Keyman-Glossary
• https://github.com/keymanapp/keyman/wiki/Backspace-and-cluster-deletion