Add description for CAMUNDA_IDENTITY_* environment variables

[maryarm]

This issue raised from a support case that customer asks about descriptions of the following environment variables:

CAMUNDA_IDENTITY_ISSUER
CAMUNDA_IDENTITY_ISSUER_BACKEND_URL
CAMUNDA_IDENTITY_BASEURL
CAMUNDA_IDENTITY_TYPE

There is already references to these variables in docs, but adding some context about each of these variables meaning and sample values can lead to a better understanding.