Added three new sections to the docs

The "Testing" section already has a few PRs about to be merged.

Will need to make stub articles for the "Sourcing content and data" section and the "Adding third-party services" section.

@calcsam I don't know if I did the "Headless CMS" section inside the "Sourcing content and data" section correctly. Also, this process is giving me pause on how many levels of infinite nesting is reasonable. There is a point at which we'd need to actually create a whole new page for a set of docs if it has enough children with grand-children and great-great-grand-children, etc!

www/src/data/sidebars/doc-links.yaml

@@ -83,19 +83,30 @@
           link: /docs/programmatically-create-pages-from-data/
         - title: Querying data with StaticQuery
           link: /docs/static-query/
-    - title: Headless CMS*
-      link: /docs/headless-cms/
+    - title: Sourcing content and data*
+      link: /docs/sourcing-content-and-data
       items:
-        - title: Netlify CMS
-          link: /docs/netlify-cms/
-        - title: WordPress*
-          link: /docs/wordpress/
-        - title: Drupal*
-          link: /docs/drupal/
-        - title: Contentful*
-          link: /docs/contentful/
-        - title: Prose*
-          link: /docs/prose/
+        - title: Filesystem*
+          link: /docs/filesystem/
+        - title: Database*
+          link: /docs/database/
+        - title: SaaS service*
+          link: /docs/saas-service/
+        - title: Private APIs*
+          link: /docs/private-apis/
+        - title: Headless CMS*
+          link: /docs/headless-cms/
+            items:
+              - title: Netlify CMS
+                link: /docs/netlify-cms/
+              - title: WordPress*
+                link: /docs/wordpress/
+              - title: Drupal*
+                link: /docs/drupal/
+              - title: Contentful*
+                link: /docs/contentful/
+              - title: Prose*
+                link: /docs/prose/
     - title: Plugins*
       link: /docs/plugins/
       items:
@@ -122,6 +133,19 @@
           link: /docs/component-css/
         - title: PostCSS*
           link: /docs/post-css/
+    - title: Testing*
+      link: /docs/testing/
+      items:
+        - title: Unit testing
+          link: /docs/unit-testing/
+        - title: Testing components with GraphQL
+          link: /docs/testing-components-with-graphql/
+        - title: End-to-end testing
+          link: /docs/end-to-end-testing/
+        - title: Testing CSS-in-JS
+          link: /docs/testing-css-in-js/
+        - title: Testing React components
+          link: /docs/testing-react-components/
     - title: Debugging
       link: /docs/debugging/
       items:
@@ -161,6 +185,15 @@
           link: /docs/linking-between-pages/
         - title: Search Engine Optimization (SEO)
           link: /docs/seo/
+    - title: Adding third-party services*
+      link: /docs/adding-third-party-services/
+      items:
+        - title: Search*
+          link: /docs/search/
+        - title: Analytics*
+          link: /docs/analytics/
+        - title: Forms*
+          link: /docs/forms/
     - title: Performance*
       link: /docs/performance/
       items: