docs(custom-controllers): start work on custom controllers

docs/pages/developing-extensions/Custom-controllers.md

@@ -2,10 +2,12 @@
 <h4 class="fw-light">Write custom controllers instead of using the premade one</h4><br/>
 
 <div class="alert alert-dark bg-body mt-3 rounded-4 border" role="alert">
-  <i class="bi bi-x mb-1" style="font-size:23px; float: left;"></i>
-  <div class="ps-3 ms-3"><b>This guide is not available yet.</b> We are working on writing a brand new set of guides, which you can contribute to <a href="https://github.com/teamblueprint/web">here</a>.</div>
+  <i class="bi bi-pin-angle-fill mb-1" style="font-size:23px; float: left;"></i>
+  <div class="ps-3 ms-3"><b>This guide is still being written.</b> You might run into unfinished explainations, incorrect instructions, outdated or misinformation.</div>
 </div><br/>
 
+Controllers allow for added functionality and flexibility to your extension's admin page. This can be useful when making configuration pages, calculating certain things beforehand, query APIs and much more.
+
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