Manage user's expectation on the freshness of the schema

[imolorhe]

Is your feature request related to a problem? Please describe.
It seems there's no clear indication to the user about how "fresh" the docs schema is. In a very dynamic environment (where the schema on the server changes a lot), this might be problematic as the user might not be aware that the schema being used in Altair is stale.

Describe the solution you'd like
Provide some visual information showing when the schema was last fetched from the server, if the schema was fetched from the server.

Additional context
Related issue: #1145