Guide: Use case driven guides for credential-stores

[PrarthonaPaul]

Use cases:

• replace cleartext passwords in credential-reference
• Encrypted expressions
• WildFly client configuration

[bstansberry]

@PrarthonaPaul Note there is also #580 which is a specific case of a use-case driven guide involving a credential-store. We should have a coherent approach.

#580 is step 3 in a multi-step series of datasource guides, so we have time to think about this before we get to that step.