Add missing definitions to Dagger CUE API

Link to the source of truth cue files.

While at it, move this page from Knowledge Base, which is a generic
category, to Core Concepts. The URL didn't change so that we don't break
existing links. FWIW /1213/dagger-cue would have been my choice.

Initially contributed as a change part of
https://github.com/dagger/dagger/pull/1758

Should be merged after https://github.com/dagger/dagger/pull/1799

Related to https://github.com/dagger/dagger/pull/1768

Signed-off-by: Gerhard Lazu <gerhard@lazu.co.uk>

website/sidebars.js

@@ -89,6 +89,7 @@ module.exports = {
         "core-concepts/secrets",
         "core-concepts/container-images",
         "core-concepts/what-is-cue",
+        "core-concepts/dagger-cue",
       ],
     },
     {
@@ -98,7 +99,6 @@ module.exports = {
       collapsed: false,
       items: [
         "knowledge-base/migrate-from-dagger-0.1",
-        "learn/api",
         "use-cases/go-docker-swarm",
       ],
     },