docs: Restructure and improve DNS documentation (#504)

* docs: Restructure and improve DNS documentation

Reorganizes DNS docs into overview, configuration, settings, and troubleshooting
guides with clearer explanations and UI-aligned terminology

* amendments from Vik's feedback

* clarified IP assignment range

* remove incorrect common mistake note

* - added diagrams
- renamed dns overview.mdx to index.mdx
- updated references/redirects
- created new spacer div class

---------

Co-authored-by: Ashley Mensah <ashley@netbird.io>

src/styles/tailwind.css

@@ -114,6 +114,10 @@
   border: 0;
 }
 
+.spacer-sm {
+  height: 5px;
+}
+
 .Toastify__close-button {
   color: var(--toastify-text-color-light) !important;
 }