TINYDOC-3541 - Add redirects for migration-from-6x and powerpaste 404s (#4235)

Add redirect entries mapping the renamed pages:
- /docs/tinymce/latest/migration-from-6x/ -> /docs/tinymce/latest/migration-from-6x-to-8x/
- /docs/tinymce/latest/powerpaste/ -> /docs/tinymce/latest/introduction-to-powerpaste/

Author: kemister85

redirects.json

@@ -2877,6 +2877,14 @@
     "location": "/docs/tinymce/latest/mathequations/",
     "redirect": "/docs/tinymce/latest/math/"
   },
+  {
+    "location": "/docs/tinymce/latest/migration-from-6x/",
+    "redirect": "/docs/tinymce/latest/migration-from-6x-to-8x/"
+  },
+  {
+    "location": "/docs/tinymce/latest/powerpaste/",
+    "redirect": "/docs/tinymce/latest/introduction-to-powerpaste/"
+  },
   {
     "location": "/docs/tinymce/latest/premium-full-featured/",
     "redirect": "/docs/tinymce/latest/full-featured-premium-demo/"