docs: extensions (#1258)

* docs: process docs to include UX Stories requirement

* docs: include note regarding different environments

* Services init

* Remove unused canny logos

* docs: add ModalService diagram

* docs: GIF of notification

* docs: add ui-services page

* docs: simplify ui services call out in the general services docs

* docs: tips and tricks for UI services

* docs: moar pages

* docs: dialog gif

* docs: services in summary/sidebar

* docs: gif examples at top of dialog and notification pages

* docs: add UIModal gif

* docs: details for Dialog Service

* docs: include usage information for ui modal service

* docs: detailed information about our UI Notification Service

* docs: services diagram

* docs: remove unused links

* docs: services example and image

* docs: stubbing extension pages

* docs: UX Stories --> User Cases

* chore: eslint fix

* docs: extensions overview + diagram

* docs: update extensions in sidebar

* docs: extension skeleton and registration info

* docs: copy of maintained extensions table partial in the extensions folder

* docs: swap out old extensions structure for new

docs/latest/extensions/lifecycle/pre-registration.md

@@ -0,0 +1,21 @@
+# Lifecylce Hook: preRegistration
+
+If an extension defines the `preRegistration` lifecycle hook, it is called
+before any modules are registered to the `ExtensionManager`.
+
+```js
+export default {
+  id: 'MyExampleExtension',
+
+  preRegistration({ servicesManager, commandsManager, configuration }) {
+    console.log('Wiring up important stuff.');
+
+    window.importantStuff = () => {
+      console.log(configuration);
+    };
+
+    console.log('Important stuff has been wired.');
+    window.importantStuff();
+  },
+};
+```