Deprecate settings.yml, and document usage (#1692)

Resolves #1524.

The outcome of our discussion in the ticket was this:

- The SQlite DB (`tinypilot.db`) should be the default place for new
settings, whereas the YAML file (`settings.yml`) should only be used in
exceptional cases, and obviously for legacy reasons.
- We favor having a single storage solution over a dual approach, as
that simplifies future decision-making, and eliminates potential
migration burden between the two storage formats.
- The SQlite DB doesn’t easily allow programmatic access right now (in
contrast to the YAML file), so we [created this ticket for removing this
hurdle](#1690).
- We [will document the parameters in `settings.yml`
separately](#1573).

This PR adds a comment in `settings.py`, marking its usage as
quasi-deprecated. It also adds some historical context, references the
SQlite DB as default storage, and lists sample exceptional cases.

I realized that we probably don’t need to add any documentation to the
SQlite DB, since there is not really an alternative to it any longer. So
by deprecating `settings.yml`, the SQlite DB is the only storage option
anyway, and there shouldn’t be any ambiguity left. Contrary to [my
initial
proposal](#1524 (comment)),
I also don’t think we should move `settings.py` up to the `app` package,
as that might mistakenly promote its usage.
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---------

Co-authored-by: Jan Heuermann <jan@jotaen.net>

app/update/settings.py

@@ -1,14 +1,29 @@
 """Manages a TinyPilot update settings file.
 
-TinyPilot currently manages some of its settings in a database and some of its
-settings in a YAML settings file. This module is a wrapper around the YAML
-settings file that allows TinyPilot code to modify it cleanly.
+This module is a wrapper around the YAML settings file (settings.yml) that
+allows TinyPilot code to modify it cleanly.
 
 Typical usage example:
 
     settings = update_settings.load()
     settings.ustreamer_desired_fps = 15
     update_settings.save(settings)
+
+Note that we consider the use of settings.yml to be deprecated nowadays. The
+default place to store settings and configuration is the SQlite database. We
+should only add new properties to settings.yml in exceptional cases, such as:
+- Settings that are supposed to be manually overridden by users
+- Settings that are closely related to already existing legacy settings
+
+Historically, settings.yml was used as we relied on Ansible to manage and
+update the device system state. The canonical way for configuration in Ansible
+is files in YAML format, so a YAML file was an obvious choice here. This
+wrapper module resides in the update package, since settings.yml was only
+processed throughout the update process initially. Over time, we also added a
+few other settings, which are independent of the update context.
+As of mid 2023, we have finished migrating away from Ansible, so there is no
+hard requirement for us to continue using the settings.yml file, except for
+backwards compatibility with existing TinyPilot installations.
 """
 
 import yaml