Simplify the first build guide

[bbhtt]

Addresses most of the things in https://github.com/flatpak/flatpak-docs/issues/230, so closes https://github.com/flatpak/flatpak-docs/issues/230

This simplifies the first build guides and reduces possible failure points.

• script is self-contained in the manifest
• build and install instructions combined to avoid running multiple commands
• Add --install-deps-from to avoid figuring out installing runtimes
• Removes creating a repo - not sure why this is in a hello world guide. Exporting it to a repo has no direct use cases since it can be installed with flatpak-builder directly and the section references more complicated topics such as appdata files, build-update, gpg-verify etc. that need not be in a hello world guide.

So I replaced it with sharing a single file bundle, which seems to be more useful in a hello world situation.

[TingPing]

Add --install-deps-from to avoid figuring out installing runtimes

There is a small gotcha here. If they follow the website to add Flathub it will be added as a system repository and --user --install-deps-from=flathub will fail.

[bbhtt]

Add --install-deps-from to avoid figuring out installing runtimes

There is a small gotcha here. If they follow the website to add Flathub it will be added as a system repository and --user --install-deps-from=flathub will fail.

I mentioned the command to remote-add --user in the top lines 7-11. To be honest, I think the website should recommend --user

[bbhtt]

I think it's good enough right now to land as it solves a bunch of issues.

Installing the remote is explicitly mentioned as a step, so I hope no one will skip that.