Migrate X86 image example from examples repo to main repo

buildstream!262 (closed) has been lying bare for quite the while now. It's an example that intends to show how to use the x86image plugin. After lots of redirection we've decided it should land here, and that we should introduce a new examples page that is linked to from the main repository.

Some reconsideration and discussion with @danielsilverstone-ct also unveiled the issue that the current example builds an initramfs out of musl+busybox and then decides to boot the very same system. This is simple, but not very copy-pastable. We should replace the booted system with something a bit more realistic, such as https://www.alpinelinux.org/ or https://www.archlinux.org/ (if we want a proper GNU system). Not freedesktop this time, because it's rather hard to bootstrap into a real system.

So the tasks:

• Add an examples page
• Make the example tutorial compliant with new examples requirements
• Make the example boot a full system, rather than just use a musl + busybox initramfs to boot a musl + busybox system
• Link to the new examples page from the main repository

This might also dig up #3 (moved), since this tutorial uses a docker source.