Add Custom Emojis endpoint to the docs. (#702)

* Add Custom Emojis endpoint to the docs.

Everyone needs to know about the most important endpoint.

* Move custom emoji API listing.

* Move custom emojis back under Instances, adding a listing to the ToC

Using-the-API/API.md

@@ -16,6 +16,7 @@ API overview
   - [Follow Suggestions](#follow-suggestions)
   - [Follows](#follows)
   - [Instances](#instances)
+    - [Custom Emojis](#custom-emojis)
   - [Lists](#lists)
   - [Media](#media)
   - [Mutes](#mutes)
@@ -456,6 +457,8 @@ Returns the current [Instance](#instance).
 
 Does not require authentication.
 
+### Custom Emojis
+
 #### Getting current instance's custom emojis:
 
     GET /api/v1/custom_emojis