feat(mkdocs.yml): move tutorials and explainers to the top-level

Now that we have some content for both sections, move them to the
top-level. This means that we have a single top-level category for each
of the diataxis quadrants.

mkdocs.yml

@@ -47,29 +47,28 @@ nav:
       - "Cloud Platform":
           - howtos/respond-to-billing-alerts.md
           - howtos/create-gcloudadmin-account.md
-  - "Learn":
-      - "Tutorials":
-          - tutorials/index.md
-          - tutorials/creating-a-python-package.md
-          - tutorials/bootstrapping-a-new-application.md
-          - tutorials/automating-gitlab-releases.md
-      - "Explanations":
-          - explanations/index.md
-          - explanations/gcloudadmin-accounts.md
-          - explanations/service-infrastructure.md
-          - explanations/identifiers.md
-          - explanations/webapp-boilerplate.md
-          - explanations/tech-lead-forum.md
-          - "Cloud Platform":
-              - explanations/gcp-budgets-alerts.md
-          - "Git/GitLab":
-              - explanations/good-issues.md
-              - explanations/reviewing-issues.md
-              - explanations/git.md
-              - explanations/gitlab-release-automation.md
-          - explanations/gke-gitlab-runners.md
-          - "Python":
-              - workflow/pypi.md
+  - "Tutorials":
+      - tutorials/index.md
+      - tutorials/creating-a-python-package.md
+      - tutorials/bootstrapping-a-new-application.md
+      - tutorials/automating-gitlab-releases.md
+  - "Explanations":
+      - explanations/index.md
+      - explanations/gcloudadmin-accounts.md
+      - explanations/service-infrastructure.md
+      - explanations/identifiers.md
+      - explanations/webapp-boilerplate.md
+      - explanations/tech-lead-forum.md
+      - "Cloud Platform":
+          - explanations/gcp-budgets-alerts.md
+      - "Git/GitLab":
+          - explanations/good-issues.md
+          - explanations/reviewing-issues.md
+          - explanations/git.md
+          - explanations/gitlab-release-automation.md
+      - explanations/gke-gitlab-runners.md
+      - "Python":
+          - workflow/pypi.md
   - "Reference":
       - reference/index.md
       - external-docs.md