Add a callout to the collection repr

**Is your feature request related to a problem? Please describe.**
I am currently restructuring the [Pangeo/ESGF CMIP6 Cloud Data](https://github.com/leap-stc/cmip6-leap-feedstock) and would like for everyone to slowly migrate their workflows to this new catalog. Now I can change the documentation of the main entry points (docs/gh repos/...) but it is impossible to hunt down every instance of the 'legacy' catalog and replace it.

**Describe the solution you'd like**
I do however have access to that legacy json file, and am wondering if there is a way to add a 'message' item, which would then be displayed as a red callout when calling the collection html repr?
That way everyone who access the legacy catalog and runs a recent intake-esm version would get alerted that there is a new catalog whenever they rerun their code.

Would this be doable/resonable?

Start by tracing the collection HTML repr and how catalog JSON fields are parsed, then check how an optional message item could reach the rendered output. Done means a legacy catalog can provide a message that recent intake-esm versions display as a red callout when the collection repr is generated, without breaking catalogs that omit it.

Written by the indexing model from the issue text.