refactor(blog): standardize blog covers (#1872)

Author: felangel

docs/src/assets/blog/future_of_dart_frog_dark.svg renamed to docs/src/assets/blog/future_of_dart_frog/cover_dark.svg

@@ -9,8 +9,8 @@ authors:
 featured: true
 cover:
   alt: Future of Dart Frog Cover
-  dark: ../../../assets/blog/future_of_dart_frog_dark.svg
-  light: ../../../assets/blog/future_of_dart_frog_light.svg
+  dark: ../../../assets/blog/future_of_dart_frog/cover_dark.svg
+  light: ../../../assets/blog/future_of_dart_frog/cover_light.svg
 tags:
   - community
 ---