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

@@ -0,0 +1,26 @@
+dagger.#Plan & {
+  client: {
+    filesystem: {
+      // ...
+    }
+    env: {
+      // ...
+    }
+  }
+  actions: {
+    deps: docker.#Build & {
+      // ...
+    }
+    test: bash.#Run & {
+      // ...
+    }
+    build: {
+      run: bash.#Run & {
+         // ...
+      }
+      contents: dagger.#Subdir & {
+        // ...
+      }
+    }
+  }
+}