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:

docs/getting-started/1200-local-dev.md

@@ -42,7 +42,7 @@ With Docker running, we are ready to download our example app and run its CI/CD
 ```shell
 git clone https://github.com/dagger/dagger
 cd dagger
-git checkout v0.2.0-beta.1
+git checkout v0.2.0-beta.2
 
 cd pkg/universe.dagger.io/examples/todoapp
 ```
@@ -123,7 +123,7 @@ With Docker Engine running, we are ready to download our example app and run its
 ```shell
 git clone https://github.com/dagger/dagger
 cd dagger
-git checkout v0.2.0-beta.1
+git checkout v0.2.0-beta.2
 
 cd pkg/universe.dagger.io/examples/todoapp
 ```

docs/getting-started/1201-ci-environment.md

@@ -7,8 +7,8 @@ displayed_sidebar: europa
 
 Dagger can be used with any CI environment (no migration required) and has two important advantages which make the overall experience less error-prone and more efficient:
 
-1. Instead of YAML you write CUE: typed configuration with built-in formatting
-2. Configuration is executed in buildkit, the execution engine at the heart of Docker
+1. Instead of YAML, you write [CUE](https://cuelang.org/) - typed configuration with built-in formatting
+2. Configuration is executed in [BuildKit](https://github.com/moby/buildkit), the execution engine at the heart of Docker
 
 This makes any CI environment with Docker pre-installed work with Dagger out of the box.
 We started with [CI environments that you told us you are using](https://github.com/dagger/dagger/discussions/1677).
@@ -32,38 +32,7 @@ values={[
 
 <TabItem value="github-actions">
 
-`.github/workflows/todoapp.yml`
-
-```yaml
-name: todoapp
-
-push:
-  # Trigger this workflow only on commits pushed to the main branch
-  branches:
-    - main
-
-# Dagger plan gets configured via client environment variables
-env:
-  # This needs to be unique across all of netlify.app
-  APP_NAME: todoapp-dagger-europa
-  NETLIFY_TEAM: dagger
-  # https://app.netlify.com/user/applications/personal
-  NETLIFY_TOKEN: ${{ secrets.NETLIFY_TOKEN }}
-  DAGGER_LOG_FORMAT: plain
-
-jobs:
-  dagger:
-    runs-on: ubuntu-latest
-    steps:
-      - name: Clone repository
-        uses: actions/checkout@v2
-
-      - name: Deploy to Netlify
-        # https://github.com/dagger/dagger-for-github
-        uses: dagger/dagger-for-github@v2
-        with:
-          workdir: pkg/universe.dagger.io/examples/todoapp
-          args: do deploy
+```yaml file=../tests/getting-started/github-actions.yml title=".github/workflows/todoapp.yml"
 ```
 
 </TabItem>

docs/learn/1213-api.md

@@ -11,7 +11,7 @@ The Dagger CUE API is the set of CUE packages released alongside the Dagger engi
 
 ### Plan definition
 
-`#Plan` defines the structure of a Dagger plan - a complete configuration executable by `dagger up`.
+`#Plan` defines the structure of a Dagger plan - a complete configuration executable by `dagger do`.
 
 ### Core types
 

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 & {
+        // ...
+      }
+    }
+  }
+}

docs/tests/getting-started/github-actions.yml

@@ -0,0 +1,30 @@
+name: todoapp
+
+push:
+  # Trigger this workflow only on commits pushed to the main branch
+  branches:
+    - main
+
+# Dagger plan gets configured via client environment variables
+env:
+  # This needs to be unique across all of netlify.app
+  APP_NAME: todoapp-dagger-europa
+  NETLIFY_TEAM: dagger
+  # Get one from https://app.netlify.com/user/applications/personal
+  NETLIFY_TOKEN: ${{ secrets.NETLIFY_TOKEN }}
+  # GitHub Actions friendly logs
+  DAGGER_LOG_FORMAT: plain
+
+jobs:
+  dagger:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Clone repository
+        uses: actions/checkout@v2
+
+      - name: Deploy to Netlify
+        uses: dagger/dagger-for-github@v2
+        # See all options at https://github.com/dagger/dagger-for-github
+        with:
+          workdir: pkg/universe.dagger.io/examples/todoapp
+          args: do deploy