[KEP] Add ResourceFlavor fallbacks KEP

[PBundyra]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

Introduces KEP for ResourceFlavor fallbacks. The details are described in-depth in KEP itself.

Which issue(s) this PR fixes:

Part of #2560

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE