Clarify KVStore behavior with Namespace in OSS

[Ben8t]

Description

https://kestra.io/docs/concepts/kv-store#namespace-binding

Raised by an OSS user, the KeyValue Store in OSS can't be used across namespaces - i.e. a kv in namespace A can't be read in a namespace B if the namespaces aren't nested in some way. Only EE version allow cross namespaces key value sharing.

Can be great to adjust the documentation by being more explicit about what's possible in OSS vs EE

"In the key value docs, there is a link to this about EE, where it claims that "By default, all namespaces are allowed".
Although I can without problems access flows across namespaces, the same doesn't seem to be the case for key_values
this is the error I get (flow in the thread): Cannot access the KV store. The 'system' namespace is neither equal to nor a descendant of 'kestra.get_kv'"
"[...] I can't access the key value pairs from that namespace. which is odd, I think. I also tested with the {{ kv('key', 'other_namespace') }} notation, which seems to have the intention to exactly serve my use case, but get the same error."

[shrutimantri]

We started with cross namespace access, but looks like later we limited the access to only the same namespace hierarchy.
As of now, we are getting the same error in cross namespace kv store access even in EE.

@Ben8t let me know whats the expected behavior here, and I will make changes to the documentation accordingly.

[Ben8t]

link to kestra-io/kestra#5037