feat: add docs for github_user_ssh_signing_key

Author: simonostendorf

website/docs/r/user_ssh_signing_key.html.markdown

@@ -0,0 +1,42 @@
+---
+layout: "github"
+page_title: "GitHub: github_user_ssh_signing_key"
+description: |-
+  Provides a GitHub user's SSH signing key resource.
+---
+
+# github_user_ssh_signing_key
+
+Provides a GitHub user's SSH signing key resource.
+
+This resource allows you to add/remove SSH signing keys from your user account.
+
+## Example Usage
+
+```hcl
+resource "github_user_ssh_signing_key" "example" {
+  title = "example title"
+  key   = file("~/.ssh/id_rsa.pub")
+}
+```
+
+## Argument Reference
+
+The following arguments are supported:
+
+* `title` - (Required) A descriptive name for the new key. e.g. `Personal MacBook Air`
+* `key` - (Required) The public SSH signing key to add to your GitHub account.
+
+## Attributes Reference
+
+The following attributes are exported:
+
+* `id` - The ID of the SSH signing key
+
+## Import
+
+SSH signing keys can be imported using their ID e.g.
+
+```
+$ terraform import github_user_ssh_signing_key.example 1234567
+```