Add enum of capabilities for Regions

[nmelehan]

This PR should display the possible values that could be included in the capabilities array of a Region.

However:

• The current developers site does not display these values with the updates in this PR.
• If the enum is moved out of the items scope and into the capabilities scope (like below), then the enum does display:

      capabilities:
        type: array
        items:
          type: string
        description: >
            A list of capabilities of this region.
        enum:
        - Linodes
        - NodeBalancers
        - Block Storage
        - Object Storage
        - Kubernetes
        example:
        - Linodes
        - NodeBalancers
        - Block Storage
        - Object Storage
        readOnly: true
        x-linode-cli-display: 3
  

  
• I believe that the above code is incorrect; the openapi docs from Swagger say: All values in an enum must adhere to the specified type. In this code, the type is array, while the elements of the enum are strings.
• It seems like we should not try to update the spec file with technically incorrect content that happens to render like we want it to. Instead, we should update the renderer so that it works with the correct spec syntax.

I'm going to close this PR, so we don't keep it open while waiting on updating the renderer, while still keeping this as a record of the work done so far