:book: Improve README.md content

[displague]

The README.md could be improved in a few ways:

• [x] move 0.3 - 1.0 CAPI guidance to docs/MIGRATION.md
• [x] add a development readme (tilt based would be nice)
• [x] Add notes about using PacketMachine directly for the purposes of managing HW Reservations (https://github.com/kubernetes-sigs/cluster-api-provider-packet/issues/223)
• [x] add notes about flavors offered
• [x] more strongly word "Getting Started" text to say the official path is in the CAPI Quick Start Guide
• [x] add links to guided Equinix Metal documentation https://metal.equinix.com/developers/guides/kubernetes-cluster-api/

[cprivitere]

When 409 is merged, we should be good to close this.