Take users directly to API YAML source via Edit on GitHub

Author: cwarnermm

site/layouts/partials/page-edit.html

@@ -1,5 +1,11 @@
 {{ if .File }}
+{{ if eq .Section "api-documentation" }}
+<a href="https://github.com/mattermost/mattermost/tree/master/api/v4/source" class="float-right edit-github">
+    Edit on GitHub
+</a>
+{{ else }}
 <a href="{{.Site.Params.ghrepo}}edit/master/site/content/{{ .File.Path }}" class="float-right edit-github">
     Edit on GitHub
 </a>
 {{ end }}
+{{ end }}