Add relevant configs of way to Preserve/Override the Authorization header in the Micro-gateway

Description:
To send the same "Authorization" header from the initial request to the back end api. if we want the original request header used for Authorization to be passed through the micro-gateway to the back-end service as require information from the JWT token on the web service  and if don't want to use different header values for this between WSO2 and the back end service.
We cant use the configuration which is mentioned in documentation[1].

In order to achieve that we have use the below configuration.

[authConfig]
  # Authorization header expected by the Micro-gateway. Can be overridden at API level using the extension
  authorizationHeader = "Authorization"
  # Remove authorization header from the backend request
  removeAuthHeaderFromOutMessage = false  

But those details were not there in the documentation[1]. Those details need to be included to the documentation[1].

Suggested Labels:

Micro-gateway-3.2.0

[1] https://mg.docs.wso2.com/en/latest/how-tos/preserve-override-headers/

Open the linked preserve/override headers documentation page and compare it with the [authConfig] settings shown in the issue. Document authorizationHeader and removeAuthHeaderFromOutMessage, including how they preserve or remove the Authorization header. Done means the page explains these settings accurately and the configuration example is clear.