Add docs for HCP Packer and Azure Compute Galleries/Shared Image Galleries

#### Community Note

Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request.
Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request.
If you are interested in working on this issue or have submitted a pull request, please leave a comment.

#### Description

Add documentation for how to use HCP Packer with Azure Compute Galleries to create parent/child (ancestry) images. While not using managed_images.

#### Use Case(s)

This will help others understand how to best utilize HCP Packer with Azure service. It seems that all of Packer's HCP docs are centered around using AWS.

Start by reviewing the existing HCP Packer documentation and its AWS-focused examples, then compare them with Azure Compute Galleries and Shared Image Galleries. Document how to create parent/child ancestry images without using managed_images. Done means users can follow the Azure workflow and understand how it differs from the existing AWS guidance.

Written by the indexing model from the issue text.