Reference the Asset microservices documentation

Asset microservices custom workers are headless Project Firefly apps.
We should add a reference to the documentation for developers looking for specific details and related walkthrough.

Link: https://docs.adobe.com/content/help/en/experience-manager-cloud-service/assets/manage/asset-microservices-configure-and-use.html

Locate the relevant App Builder documentation page and review how related external references are presented. Add a reference to the Asset microservices documentation at the supplied URL, and verify that the link is visible and correctly formatted.

Written by the indexing model from the issue text.