chore: add multi auth back to v1 previous (#7335)

Author: khatruong2009

src/fragments/lib-v1/auth/native_common/signin_next_steps/common.mdx

@@ -2,40 +2,54 @@ After a user has finished signup, they can proceed to sign in. Amplify Auth sign
 
 import ios0 from '/src/fragments/lib-v1/auth/ios/signin_next_steps/10_signin.mdx';
 
-<Fragments fragments={{ swift: ios0 }} />
+import flutter0 from '/src/fragments/lib-v1/auth/flutter/signin_next_steps/10_signin.mdx';
+
+<Fragments fragments={{ swift: ios0, flutter: flutter0 }} />
 
 ### Confirm signin with SMS MFA
 
 import ios1 from '/src/fragments/lib-v1/auth/ios/signin_next_steps/20_confirm_sms_mfa.mdx';
 
-<Fragments fragments={{ swift: ios1 }} />
+import flutter1 from '/src/fragments/lib-v1/auth/flutter/signin_next_steps/20_confirm_sms_mfa.mdx';
+
+<Fragments fragments={{ swift: ios1, flutter: flutter1 }} />
 
 ### Confirm signin with custom challenge
 
 import ios2 from '/src/fragments/lib-v1/auth/ios/signin_next_steps/30_confirm_custom_challenge.mdx';
 
-<Fragments fragments={{ swift: ios2 }} />
+import flutter2 from '/src/fragments/lib-v1/auth/flutter/signin_next_steps/30_confirm_custom_challenge.mdx';
+
+<Fragments fragments={{ swift: ios2, flutter: flutter2 }} />
 
 ### Confirm signin with new password
 
 import ios3 from '/src/fragments/lib-v1/auth/ios/signin_next_steps/40_confirm_new_password.mdx';
 
-<Fragments fragments={{ swift: ios3 }} />
+import flutter3 from '/src/fragments/lib-v1/auth/flutter/signin_next_steps/40_confirm_new_password.mdx';
+
+<Fragments fragments={{ swift: ios3, flutter: flutter3 }} />
 
 ### Reset password
 
 import ios4 from '/src/fragments/lib-v1/auth/ios/signin_next_steps/50_reset_password.mdx';
 
-<Fragments fragments={{ swift: ios4 }} />
+import flutter4 from '/src/fragments/lib-v1/auth/flutter/signin_next_steps/50_reset_password.mdx';
+
+<Fragments fragments={{ swift: ios4, flutter: flutter4 }} />
 
 ### Confirm Signup
 
 import ios5 from '/src/fragments/lib-v1/auth/ios/signin_next_steps/60_confirm_signup.mdx';
 
-<Fragments fragments={{ swift: ios5 }} />
+import flutter5 from '/src/fragments/lib-v1/auth/flutter/signin_next_steps/60_confirm_signup.mdx';
+
+<Fragments fragments={{ swift: ios5, flutter: flutter5 }} />
 
 ### Done
 
 import ios6 from '/src/fragments/lib-v1/auth/ios/signin_next_steps/70_done.mdx';
 
-<Fragments fragments={{ swift: ios6 }} />
+import flutter6 from '/src/fragments/lib-v1/auth/flutter/signin_next_steps/70_done.mdx';
+
+<Fragments fragments={{ swift: ios6, flutter: flutter6 }} />