Add tier badges to Source Code MR docs

Update the page titles with tier badges for Source Code-related MR docs

doc/user/project/merge_requests/approvals/rules.md

@@ -5,7 +5,7 @@ info: "To determine the technical writer assigned to the Stage/Group associated
 type: reference, concepts
 ---
 
-# Merge request approval rules
+# Merge request approval rules **(FREE)**
 
 Approval rules define how many [approvals](index.md) a merge request must receive before it can
 be merged, and which users should do the approving. You can define approval rules:

doc/user/project/merge_requests/approvals/settings.md

@@ -5,7 +5,7 @@ info: "To determine the technical writer assigned to the Stage/Group associated
 type: reference, concepts
 ---
 
-# Merge request approval settings
+# Merge request approval settings **(FREE)**
 
 You can configure the settings for [merge request approvals](index.md) to
 ensure the approval rules meet your use case. You can also configure