[docs] Document examples of different JWT providers and their configurations

It would be great to have a source of truth of how to configure and get Hasura to work with different JWT providers.

1. What the `HASURA_GRAPHQL_JWT_SECRET` value should be
2. How to customize the role and user-id claims:
a. Option 1: Use hasura JWT claim customization (`claims_map`) which I think most of our users don't even know about!
b. Option 2: Use the identity providers customization feature (eg: auth0 hooks, firebase cloud functions)

I had 3 ideas on where this source of truth could be!

### Idea - 1
A nice docs page where the Hasura team and the community can keep contributing examples that have the subsections above

### Idea - 2
Enhance the hasura.io/jwt-config widget and make it oss so that becomes the single point of reference where everyone can contribute

### Idea - 3
Make this a feature on Hasura Cloud in the env var section (or embed the widget from Idea -2!)

Just some thoughts, but something that would be super helpful and a great time saver.

Some identity providers we commonly run in to:
1. Firebase
2. Auth0
3. Okta
3. Cognito
4. KeyCloak
5. Google Cloud Identity Provider
6. Azure AD
7. Fusion Auth

No file or test is named. Start by reviewing the HASURA_GRAPHQL_JWT_SECRET, claims_map, and hasura.io/jwt-config references, then resolve whether the source of truth should be a docs page, an open-source widget, or a Hasura Cloud feature. Done would require an agreed approach and documented configurations for the listed identity providers.

Written by the indexing model from the issue text.