Docs - Reverse proxy - Caddy v2 installation notes

Related issue: n/a

This Merge Request includes:

• Tests
• A changelog fragment (cf https://docs.funkwhale.audio/contributing.html#changelog-management)

Updated documentation to cover the configuration of Caddy v2, which was released in May. There are breaking changes in v2, including to the proxy directive mentioned in the documentation. See Upgrade Guide - proxy

There are now two methods for deploying the configuration: Caddyfile and API. I thought that if people needed to follow a tutorial on Funkwhale for setting up Caddy then the API implementation is out of scope. But I guess there's no harm in adding it if people think it's useful.

The original configuration has been left as v1 will still receive fixes for approximately 6 months.