Extract snippets from docs into separate files

So that we get auto-formatting and syntax checking in our code editor.
The only snippets which have not been extracted are either terminal
output, or file fragments (e.g. CUE) which are not valid standalone files.

Resolves https://github.com/dagger/dagger/issues/1715

While at it, do a few fly-by improvements:
- beta.1 -> beta.2
- add CUE & BuildKit links
- up -> do

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

docs/core-concepts/1202-plan.md

@@ -15,33 +15,7 @@ This plan is the entrypoint for everything that runs within a pipeline. The simp
 
 This is our **Getting Started** example app plan structure:
 
-```cue
-dagger.#Plan & {
-  client: {
-    filesystem: {
-      // ...
-    }
-    env: {
-      // ...
-    }
-  }
-  actions: {
-    deps: docker.#Build & {
-      // ...
-    }
-    test: bash.#Run & {
-      // ...
-    }
-    build: {
-      run: bash.#Run & {
-         // ...
-      }
-      contents: dagger.#Subdir & {
-        // ...
-      }
-    }
-  }
-}
+```cue file=../tests/core-concepts/plan/structure.cue.fragment
 ```
 
 When the above plan gets executed via `dagger do build`, it produces the following output: