Prepare for new docs, remove user docs as they move to own repo (#4819)

doc/_themes/pretix_theme/layout.html

@@ -115,6 +115,7 @@
             {%- if logo or logo_url %}
               <img src="{{ _logo_url }}" class="logo" alt="{{ _('Logo') }}"/>
             {%- endif %}
+            Developer Documentation
           </a>
 
           {% include "searchbox.html" %}

doc/_themes/pretix_theme/static/css/pretix.css

@@ -5183,7 +5183,7 @@ div[class^='highlight'] pre {
 
 .wy-side-nav-search > a img.logo, .wy-side-nav-search .wy-dropdown > a img.logo {
     display: block;
-    margin: 0 auto;
+    margin: 0 auto 10px;
     height: 50px;
     width: auto;
     border-radius: 0;