Update "Manage" to "Foundations" in backend tools

These files will incur a longer pipeline, so let's keep them separate
from the other docs changes.

doc/api/graphql/reference/index.md

 ---
-stage: Manage
+stage: Foundations
 group: Import and Integrate
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://handbook.gitlab.com/handbook/product/ux/technical-writing/#assignments
 ---

tooling/graphql/docs/helper.rb

@@ -55,7 +55,7 @@ module Helper
         def auto_generated_comment
           <<-MD.strip_heredoc
             ---
-            stage: Manage
+            stage: Foundations
             group: Import and Integrate
             info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://handbook.gitlab.com/handbook/product/ux/technical-writing/#assignments
             ---