Add kubernetes compatibility matrix to docs

[samox73]

Type of question

Are you asking about community best practices, how to implement a specific feature, or about general context and help around the operator-sdk?

Improving the OLM documentation.

Question

What did you do? Search the docs for compatible kubernetes versions for specific OLM versions.

What did you expect to see? A compatibility matrix or something similar.

What did you see instead? Under which circumstances? No documentation.