[FEATURE REQ] Add description of how to override the default TokenCredential bean

**Is your feature request related to a problem? Please describe.**
Transferred from https://github.com/microsoft/spring-cloud-azure/issues/989

In Spring Cloud Azure 4.x/5.x we provided a default `TokenCredential` bean named `AzureContextUtils.DEFAULT_TOKEN_CREDENTIAL_BEAN_NAME` and users could provide their own bean with this name to override the default one. We should mention it in our doc.

**Describe the solution you'd like**
Add a section in our refdoc on how to use it.

**Describe alternatives you've considered**
N/A

**Additional context**
N/A

**Information Checklist**

- [x] Description Added
- [x] Expected solution specified