Improve how we show Butane example and add basic/ready-to-use Ignition examples

[travier]

We only show the example for the Butane very late in the producing an Ignition config example: https://docs.fedoraproject.org/en-US/fedora-coreos/producing-ign/#_writing_the_butane_config

We should show Butane examples earlier and maybe inline basic ones in all the provisioning pages.

Another path is to also provide ready-to-use Ignition configs that use a default password for the core user for example or a placeholder SSH key so that users that only want to give Fedora a quick test can provision their systems by copy/pasting the Ignition config directly, optionaly replacing the SSH key.

This would make the first experience easier as it would not require the user to setup Butane.

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Related to: coreos/fedora-coreos-tracker#2006

[travier]

We should also add an example that sets the ssh key for the core user to the one set for a given account on GitHub.

[hrismarin]

I can start working on this by moving the example upper in the page.

I also think it would be useful to add password_hash to the example, explaining that by default it can only be used for local authentication at the console.