Explore making cmdeploy/chatmaild pip-installable

right now, the only way to setup a chatmail relay is via checkout of the relay repository.

Two base reasons to consider a pip-installable chatmaild and cmdeploy:

- On can not easily use the chatmail-metadata service in own email server setups. See https://codeberg.org/kassner/chatmail-container for how someone made it work with their otherwise classic email setup, based on a Fediverse-discussion https://phpc.social/@kassner/115340064265140044

- One can not do "pip install cmdeploy" and start using the `cmdeploy` command line interface just so. "pip" is a relatively well known way to install python packages and there is nothing inherently hard about making it work.

Concerns to sort out:

- versioning of chatmaild and cmdeploy (the latter probably depending on the former)
- how to generate default web pages (instead of relying on www folder), and how to do configure/do custom ones
- does a pip-installable cmdeploy interfere with the ongoing docker PRs?
- we probably still want to support installing via repository checkout

Note that this issue is only for raising concerns and discussing / tracking whether to go for it.
If so, i'd create an issue with an implementation plan, and close this one here.