docs: migration guide from equinix.metal to equinix.cloud

[t0mk]

I gpted the migration guide. Fixes #122

Let's discuss the form and content.

Rendered guide at
https://github.com/equinix-labs/ansible-collection-equinix/blob/migration_guide/MIGRATION.md

@displague What do you think?

[displague]

I think I see some problems in the table of resources. It would be good to double check that all of the resources from the Metal collection are listed with their new equivalent. Perhaps we can link to the open issue for any modules that don't have an equivalent.

It would also be helpful to show a small example that takes advantage of the most common modules. How would this example be translated to the new collection.

[t0mk]

@displague I looked through the table, sorted it and removed one duplicate.

I am not keen to do an example of old and new style. It's a lot of work to create working example of the old collection, and it would be a random setup, not guaranteed to be actually useful. If we have an issue where a user will struggle with migration, I'm willing to do it. At this point, I would just leave it as it is, the table itself is a good reference for changing playbook from old to new style.

[github-actions]

This PR is included in version 0.4.0 🎉