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/deployment/index.md

@@ -35,8 +35,7 @@ benefits, but comes at the cost of time and complexity. Some benefits include:
 
 _Today:_
 
-- Leverage [extensions](/advanced/extensions.md) to drop-in powerful new
-  features
+- Leverage [extensions](/extensions/index.md) to drop-in powerful new features
 - Add routes and customize the viewer's workflow
 - Finer control over styling and whitelabeling
 

docs/latest/deployment/recipes/embedded-viewer.md

@@ -98,7 +98,7 @@ extension enabled here][whole-slide-ext-demo] ([source code][ext-code-sandbox])
 and [without it here][whole-slide-base-demo] ([source code][code-sandbox]).
 
 You can read more about extensions and how to create your own in our
-[extensions guide](/advanced/extensions.md).
+[extensions guide](/extensions/index.md).
 
 #### FAQ