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.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ROADMAP.md

@@ -69,8 +69,8 @@ external interfaces. See `docs/architecture/structure.canvas` → *Backend – M
       a polling coordinator that deduplicates, `x-secret` parameters the editor
       renders as a secret picker, and health reporting. Connectors are installed
       packages found through the `fluksio.node_types` entry point group; the
-      contract is documented in `docs/connectors/` with a working skeleton at
-      `connectors/skeleton/`. The registry follows later
+      contract is documented at `docs/reference/connector-contract.md` with a
+      working skeleton at `connectors/skeleton/`. The registry follows later
 - [x] First real connectors written against that contract from outside the engine:
       WF-RAC aircon, calendar, UniFi presence and Art-Net, in `connectors/`.
       Built by `make connectors` and installed into the image. Calendar and