Integration guide for Caddy Proxy

[dadrus]

Preflight checklist

• I agree to follow this project's Code of Conduct.
• I have read and am following this repository's Contribution Guidelines."
• I have discussed this feature request with the community.

Describe the background of your feature request

There are already many integration guides for different proxies written, it would be however helpful for people considering using heimdall to extend the list and have a guide addressing integration with Caddy Proxy part of it

Describe your idea

Write a new guide.