docs(docs): add intervals_merge-formula.svg illustration and redirect for problem 57

apps/docs/redirects.ts

@@ -18,6 +18,7 @@ export default [
   { from: "/53", to: "/greedy/maximum-subarray" },
   { from: "/54", to: "/matrix-index-math/spiral-matrix" },
   { from: "/55", to: "/greedy/jump-game" },
+  { from: "/57", to: "/intervals/insert-interval" },
   { from: "/62", to: "/multi-d-grid-dp/unique-paths" },
   { from: "/64", to: "/multi-d-grid-dp/minimum-path-sum" },
   { from: "/70", to: "/1d-dynamic-programming/climbing-stairs" },