Add statelessness doc for users

[davidjharder]

Description

Statelessness doc for users.

Part of #400 and #305

Submitter Checklist

• Squashed commits with git rebase -i (if needed)

[TraceyC77]

This is a really good start. I would recommend having a warning / note box at the top of "Using statelessness" that if the user adds a custom config, it is their responsibility to make sure it still works after system updates.

I'd also recommend changing this subhead to "Customizing configurations" or such, since that's more relatable to the average user.
-or- separate out the topics. This doc for describing statelessness. The other doc (Configuration File Locations) can describe which configs should be edited to customize software. I'll write something up in the other one and we / the team can decide what looks best.

[TraceyC77]

samba, acupsd

Found this in /etc/, used by the APC UPS daemon (not installed by default, in the repos)

 ❯ head /etc/changeme                                                                                                                         [12:43:38]
#!/bin/sh
#
# This shell script if placed in /etc/apcupsd
# will be called by /etc/apcupsd/apccontrol when apcupsd
# detects that the battery should be replaced.
# We send an email message to root to notify him.

[davidjharder]

In my mind, this list is for things that a typical user might change. I don't want to make an exhaustive list of all the configs we ship

For pam and ssl, I'm skeptical we should point users at these configs without some sort of big flashing danger sign

See #409

[davidjharder]

Note to self: add the proper toml frontmatter

[TraceyC77]

It's been quite a while since this was updated. Is it still in progress? Thanks.