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>

typography.js

@@ -203,8 +203,8 @@ module.exports = ({ theme }) => ({
       },
       h4: {
         color: 'var(--tw-prose-headings)',
-        fontSize: theme('fontSize.sm')[0],
-        ...theme('fontSize.sm')[1],
+        fontSize: theme('fontSize.base')[0],
+        ...theme('fontSize.base')[1],
         fontWeight: '600',
         marginTop: theme('spacing.8'),
         marginBottom: theme('spacing.2'),