Publish the connector contract and authoring guide

Adds reference/connector-contract.md and code/connectors.md to the site, with
the node-type reference finally linking the contract it describes. Strict
build passes.

docs/reference/node-types.md

@@ -296,6 +296,9 @@ engine does not speak is ignored rather than half-loaded. Installing or
 upgrading one takes effect on the next engine restart, because Python does not
 re-import a changed module and a rescan would promise more than it delivers.
 
+The contract itself is in [The connector contract](connector-contract.md), and
+[Writing a connector](../code/connectors.md) walks through building one.
+
 ## See also
 
 - [Payload types](payload-types.md) — what a port may carry