Add new configuration document for connection slack

docs/reference/config-files/connection/trello.md

@@ -0,0 +1,35 @@
+---
+title: trello
+sidebar_label: trello
+---
+
+# trello
+
+The `trello` connection can be used to access Trello resources.
+
+```hcl
+connection "trello" "my_trello" {
+  api_key = "a25ad2e..."
+  token   = "ATTAb179ea..."
+}
+```
+
+## Arguments
+
+| Name      | Type   | Required? | Description |
+| --------- | ------ | --------- | ----------- |
+| `api_key` | String | Optional  | API key     |
+| `token`   | String | Optional  | API token   |
+
+All arguments are optional, and a `trello` connection with no arguments will behave the same as the [default connection](#default-connection).
+
+## Default Connection
+
+The `trello` connection type includes an implicit, default connection (`connection.trello.default`) that will be configured to set the `api_key` to the `TRELLO_API_KEY` environment variable and the `token` to the `TRELLO_TOKEN` environment variable.
+
+```hcl
+connection "trello" "default" {
+  api_key = env("TRELLO_API_KEY")
+  token   = env("TRELLO_TOKEN")
+}
+```