Revise Europa Docs - Core Concepts - It all starts with a plan

Thank you Tanguy for the "fresh pair of eyes" perspective.

Thank you Tom for the link suggestions.

Supersedes https://github.com/dagger/dagger/pull/1847

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

docs/tests/core-concepts/plan/structure.cue.fragment

@@ -1,3 +1,8 @@
+// ...
+// A plan has pre-requisited that we cover below.
+// For now we focus on the dagger.#Plan structure.
+// ...
+
 dagger.#Plan & {
   client: {
     filesystem: {
@@ -22,5 +27,8 @@ dagger.#Plan & {
         // ...
       }
     }
+    deploy: netlify.#Deploy & {
+      // ...
+    }
   }
 }