Update documentation about `hub.config`

[consideRatio]

I think the for hub.config should be refined to:

• list exceptions when using it causes problem, like hub.baseUrl
• warn or error if known exceptions are configured via NOTES.txt
• suggest that its fine to use for other things except for the exception

Context

values.schema.yaml entry defining the docs for hub.config

`hub.config` reference docs rendered

[consideRatio]

From @manics in #3266 (comment)

I think we can either say

• use hub.config where there is no explicit chart config (easiest for us)
• or list the properties than can be set with hub.config (more work as we need to keep it up to date)

[minrk]

Yeah, I think in general, hub.config should link to the jupyterhub api reference, which contains docs for configuration, and we should only cover what folks shouldn't put in there. In general, I think most config should be in this passthrough going forward, unless z2jh needs to handle it specially.

It may be relevant to also link to the config docs for each of the bundled packages, e.g. kubespawner and oauthenticator