Add goreleaser and terraform registry compatible docs

[Jubblin]

• Ensure that provider is installable via the registry with terraform 0.13

• Ensure documentation is available on https://registry.terraform.io/providers/terra-farm/xenserver/latest

• Remove and replace references to the old resource/data source docs

• Update installation instructions and terraform 0.13 migration

• Add goreleaser and terraform registry compatible docs

• Add index.md doc

• Update README

• Remove legacy docs