Add Boundary product directory to contribute.md (#1470)

Adds Boundary to the product directory in the contribution guide.

Author: Dan-Heath

docs/contribute.md

@@ -36,6 +36,7 @@ The following table lists the content directories for products currently availab
 
 | Product                    | Directory                              | Versioned? |
 | :------------------------- | :------------------------------------- | :--------: |
+| Boundary                   | `./content/boundary`                   | ✅    |
 | Consul                     | `./content/consul`                     | ✅    |
 | HCP                        | `./content/hcp-docs`                   | ❌   |
 | HCP Vault Dedicated        | `./content/hcp-docs`                   | ❌   |