Update Overhaul: Document update behaviour and architecture

[jotaen4tinypilot]

We should extract the relevant parts of our update overhaul document, and turn them into something permanent that lives inside this repository. Most importantly, that’s an overview of the update architecture and the basic update flow, targeted at readers who want to familiarize themselves with how the update mechanism generally works, and why we have set it up the way it is.

We should also specify how the update behaviour is supposed to work exactly, e.g. that it works on both clean devices, but also on ones with an existing installation. For the latter, it’s important to note that it preserves any custom settings in /home/tinypilot/settings.yml.

The documentation should include a brief guide about how to work with the bundler during development.

[jotaen4tinypilot]

I’m currently thinking a good place to give an overview of the update flow could be to add a new section in ARCHITECTURE.md. If we wanted to go more into details, we could add a README.md to the /bundler folder.

While we are not in a rush to write everything up, I think it would be good to have the documentation finished by the end of the project, so that we don’t have to come back to this at a later point.

We could create a brief outline in bullet points first, so that we can discuss the scope, structure, and aspects we want to cover, and then keep working on it “on the side” throughout August.