Add documentation for Azure AD as 3rd party Key Manager

[viduras]

Description:
We introduced third-party Key Manager support for Azure AD as 3rd party Key Manager back in 2022 (https://github.com/wso2-enterprise/wso2-apim-internal/issues/970), however, there's no documentation added 4.1.0 and 4.2.0 to follow for setting this up.

Suggested Labels:

Suggested Assignees:

Affected Product Version:
v 4.1 / 4.2

OS, DB, other environment details and versions:

Steps to reproduce:

Related Issues:

https://github.com/wso2-enterprise/wso2-apim-internal/issues/7845
https://github.com/wso2-enterprise/wso2-apim-internal/issues/970