Port to GitHub actions & publish to Terraform registry (#39)

* Port to GitHub actions

* Fix

* Make docs terraform registry compatible

docs/resources/bitbucketserver_banner.md

@@ -0,0 +1,25 @@
+# Resource: bitbucketserver_banner
+
+Manage the announcement banner, updating as required.
+
+## Example Usage
+
+```hcl
+resource "bitbucketserver_banner" "main" {
+  message = "Bitbucket is down for maintenance\n*Save your work*"
+}
+```
+
+## Argument Reference
+
+* `message` - Required. Information to display to the user. Markdown supported.
+* `enabled` - Optional. Turn the announcement banner on/off. Default `true`.
+* `audience` - Optional. Set the audience for the announcement. Must be one of `ALL` or `AUTHENTICATED`. Default `ALL`.
+
+## Import
+
+Import the banner:
+
+```
+terraform import bitbucketserver_banner.main banner
+```