Add a resource for webhook (#40)

Authored-by: Raul Barreto <raul.barreto@redbull.com>

docs/resources/bitbucketserver_repository_webhook.md

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+# Resource: bitbucketserver_repository_webhook
+
+Manage a repository level hook. Extends what Bitbucket does every time a repository changes, for example when code is pushed or a pull request is merged.
+
+## Example Usage
+
+```hcl
+resource "bitbucketserver_project" "main" {
+  key  = "MYPROJ"
+  name = "my-project"
+}
+
+resource "bitbucketserver_repository" "main" {
+  project = bitbucketserver_project.test.key
+  name    = "repo"
+}
+
+resource "bitbucketserver_repository_webhook" "main" {
+  project     = bitbucketserver_project.test.key
+  repository  = bitbucketserver_repository.test.slug
+  name        = "google"
+  webhook_url = "https://www.google.com/"
+  events      = ["repo:refs_changed"]
+  active      = true
+}
+```
+
+## Argument Reference
+
+* `project` - Required. Project Key the repository is contained within.
+* `repository` - Required. Repository slug to enable hook for.
+* `name` - Required. Name of the webhook.
+* `webhook_url` - Required. The URL of the webhook.
+* `events` - Required. A list of events to trigger the webhook url.
+* `active` - Optional. Enable or disable the webhook. Default: true
+
+## Attribute Reference
+
+* `webhook_id` - The webhook id.
+
+## Import
+
+Import a user reference using the project key, repository name and webhook name.
+
+```
+terraform import bitbucketserver_repository_webhook.main MYPROJ/repo/google
+```