Add schema keywords in lockfile

This is a 2.0 change introduced in iterative/dvc#5222

content/docs/user-guide/project-structure/pipelines-files.md

@@ -252,12 +252,14 @@ each stage name after a `@`. The final stages generated by the `foreach` syntax
 are saved to `dvc.lock`:
 
 ```yaml
-cleanups@raw2 labels2:
-  cmd: echo "raw2 labels2"
-cleanups@raw1:
-  cmd: echo "raw1"
-cleanups@labels1:
-  cmd: echo "labels1"
+schema: '2.0'
+stages:
+  cleanups@raw2 labels2:
+    cmd: echo "raw2 labels2"
+  cleanups@raw1:
+    cmd: echo "raw1"
+  cleanups@labels1:
+    cmd: echo "labels1"
 ```
 
 For lists containing complex values (e.g. dictionaries), the substitution
@@ -278,6 +280,7 @@ stages:
 
 ```yaml
 # dvc.lock
+schema: '2.0'
 stages:
   train@0:
     cmd: python train.py 3 10
@@ -307,6 +310,7 @@ stages:
 
 ```yaml
 # dvc.lock
+schema: '2.0'
 stages:
   build@uk:
     cmd: python train.py 'uk' 3 10
@@ -385,6 +389,7 @@ DVC will maintain a `dvc.lock` file for each `dvc.yaml`. Their purposes include:
 Here's an example:
 
 ```yaml
+schema: '2.0'
 stages:
   features:
     cmd: jupyter nbconvert --execute featurize.ipynb