add documentation for gke_hub_feature: Anthos Service Mesh

[modular-magician]

add documentation for gke_hub_feature: Anthos Service Mesh

closes hashicorp/terraform-provider-google#11462

tested deployment locally

scottsuarez-macbookpro2:04-27-22 scottsuarez$ terraform apply

An execution plan has been generated and is shown below.
Resource actions are indicated with the following symbols:
  + create

Terraform will perform the following actions:

  # google_gke_hub_feature.feature will be created
  + resource "google_gke_hub_feature" "feature" {
      + create_time    = (known after apply)
      + delete_time    = (known after apply)
      + id             = (known after apply)
      + location       = "global"
      + name           = "servicemesh"
      + project        = (known after apply)
      + resource_state = (known after apply)
      + state          = (known after apply)
      + update_time    = (known after apply)
    }

Plan: 1 to add, 0 to change, 0 to destroy.

Do you want to perform these actions?
  Terraform will perform the actions described above.
  Only 'yes' will be accepted to approve.

  Enter a value: yes

google_gke_hub_feature.feature: Creating...
google_gke_hub_feature.feature: Creation complete after 3s [id=projects/scottvalley/locations/global/features/servicemesh]

Apply complete! Resources: 1 added, 0 changed, 0 destroyed.
scottsuarez-macbookpro2:04-27-22 scottsuarez$ terraform destroy
google_gke_hub_feature.feature: Refreshing state... [id=projects/scottvalley/locations/global/features/servicemesh]

An execution plan has been generated and is shown below.
Resource actions are indicated with the following symbols:
  - destroy

Terraform will perform the following actions:

  # google_gke_hub_feature.feature will be destroyed
  - resource "google_gke_hub_feature" "feature" {
      - create_time    = "2022-05-23T17:55:03.891199942Z" -> null
      - id             = "projects/xxxxx/locations/global/features/servicemesh" -> null
      - labels         = {} -> null
      - location       = "global" -> null
      - name           = "servicemesh" -> null
      - project        = "xxxx" -> null
      - resource_state = [
          - {
              - has_resources = false
              - state         = "ACTIVE"
            },
        ] -> null
      - state          = [] -> null
      - update_time    = "2022-05-23T17:55:04.372970889Z" -> null
    }

Plan: 0 to add, 0 to change, 1 to destroy.

Do you really want to destroy all resources?
  Terraform will destroy all your managed infrastructure, as shown above.
  There is no undo. Only 'yes' will be accepted to confirm.

  Enter a value: yes

google_gke_hub_feature.feature: Destroying... [id=projects/scottvalley/locations/global/features/servicemesh]
google_gke_hub_feature.feature: Destruction complete after 1s

Destroy complete! Resources: 1 destroyed.

Release Note Template for Downstream PRs (will be copied)

Derived from GoogleCloudPlatform/magic-modules#6080