`pages/KEY_VALUE_STORE_SCHEMA.md` TODOs

[netmilk]

Moved from pages/[KEY_VALUE_STORE_SCHEMA.md

• Finalize this text, keep collections for now
• What is kv-store schema is used by Actor to define structure of key-value store it operates on,
  but the developer defines a non-compatible record group for "INPUT" prefix?
  Maybe the default kv-stores should be created with a default record group to cover the "INPUT" prefixes
  and give them JSON types. Then, we'd never need to worry about existing records.
  But it's a breaking change for some Actors... maybe we can only do this for V2 Actors with Actor file...
  ... it's getting quite complicated.
• What if there's a conflict between record groups?
  Shall we consider the first one matching as if the file is valid for schema?

... just add a note that these conflicting situations are unspecified behavior