Add app management related information to AppCenter on first launch

### Problem

There is too little information during Onboarding regarding what app formats and sources are supported, which are not and which are recommended. There is also almost no easily accessible information on what the differences are, how and what to sideload.

In general it shouldn't be expected from the user to have to dig through potentially outdated or unverified information on such a core part of an operating system. While the current Onboarding touches on the topic a bit and offers a link to Flathub, this is not enough explanation for someone new to Linux.

This lack of information is leading to users resorting to community subreddits or forums which usually have objectively bad advice such as recommending "just using apt like people have always done" or dropping elementary altogether, calling it not user-friendly (enough) which is obviously harmful for the project.

### Proposal

I suggest adding an onboarding screen, explaining or listing the following:
* what Flatpak is & why elementary chooses it over alternatives
* how to manage permissions *(I believe info about this is already included in the current onboarding, however, it's not detailed enough)*
* what remote elementary uses by default and why Flathub, for example, isn't there
* a few other remotes that the user could add to their system
* how to add such remotes
* how to know if a remote is trustworthy
* why they shouldn't sideload deb files and when to use deb files from the repositories

Obviously all of this is debatable and there is also the question of how this would get implemented UX-wise. Anyhow, adding all of this information would probably solve elementary's biggest issue for new users which is finding apps.

### Prior Art (Optional)

_No response_