Added Table of Contents to long documentation pages (#13890)

* Redo #13836 with new shortcode
(https://gitea.com/gitea/theme/pulls/90)

* add Api Usage

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

docs/content/doc/advanced/customizing-gitea.en-us.md

@@ -45,6 +45,10 @@ environment variable; this can be used to override the default path to something
 
 **Note:** Gitea must perform a full restart to see configuration changes.
 
+# Table of Contents
+
+{{< toc >}}
+
 ## Serving custom public files
 
 To make Gitea serve custom public files (like pages and images), use the folder