Generating provider documentation

[t0mk]

This is work on generating docs with https://github.com/hashicorp/terraform-plugin-docs

I started with resource metal_volume as an example of how it could look, relevant diff in 2de103c

fixes equinix/terraform-provider-equinix#201

(it was necessary to update tf-plugin-sdk, maybe I could do it in a separate PR, in order to get better PR diff for review)

[displague]

Have a look at this thread, tinkerbell/terraform-provider-tinkerbell#20 (comment) and this commit tinkerbell/terraform-provider-tinkerbell@1d2377d.

I think this provides a cleaner approach to introducing this bit of tooling without adding dozens of new requirements.

[t0mk]

@displague thanks, I will take a look

[ocobles]

The issue linked to this PR has been transferred to equinix/terraform-provider-equinix: equinix/terraform-provider-equinix#201

This draft is outdated (vendor folder removed, go sdk version updated, docs md files renamed), and according to the conversation in this thread tools.go approach must be replaced with go.tools.mo. Therefore, the migration to the new provider repository introduces more complexity than a fresh start.