Add Caddy 2 example (#7463)

The specific headers that are passed using this new configuration format
are Host and X-Forwarded-For, which should be all that's required.

Note that for production another matcher should be added in the first
section to properly handle the base_url lookup:
reverse_proxy /.well-known/matrix/* http://localhost:8008



Signed-off-by: Jeff Peeler <jpeeler@gmail.com>