contents: redirect some popular old paths to website

algorithms/README.md

@@ -0,0 +1,3 @@
+# Algorithms
+
+The algorithms contents have been moved to the [website](https://yangshun.github.io/tech-interview-handbook/algorithms/algorithms-introduction).

preparing/README.md

@@ -0,0 +1,3 @@
+# Preparing
+
+The cheatsheet contents have been moved to the [website](https://yangshun.github.io/tech-interview-handbook/coding-round-overview).