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>

next.config.mjs

@@ -51,12 +51,7 @@ const nextConfig = {
             },
             {
                 source: '/docs/how-to-guides/nameservers',
-                destination: '/manage/dns',
-                permanent: true,
-            },
-            {
-                source: '/docs/how-to-guides/nameservers',
-                destination: '/manage/dns',
+                destination: '/manage/dns/nameserver-groups',
                 permanent: true,
             },
             {