Document the new extra specs features

[gabriel-samfira]

As part of https://github.com/cloudbase/garm/pull/143, we also added the ability to override the default runner installation template for providers that leverage the garm-provider-common package.

We need to document that feature.

[gabriel-samfira]

That change also adds the ability to insert pre_install_scripts for Linux runners, which is also important to document.

[gabriel-samfira]

This should be documented as part of each provider that decided to implement the functionality.