DC-3707 Better-Auth Guide (#6969)

* better auth guide created

* sentence casing

* cover image added

* Optimised images with calibre/image-actions

* updates

* fix: add betterauth in cspell

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Ankur Datta <64993082+ankur-arch@users.noreply.github.com>

Author: 3 people

cSpell.json

@@ -93,7 +93,8 @@
     "serverful",
     "autoscale",
     "Tabnine",
-    "windsurfrules"
+    "windsurfrules",
+    "betterauth"
   ],
   "ignoreWords": [
     "Aiven",