docs(docs): add new problem documentation pages and source selector component

apps/docs/content/system-design/index.mdx

@@ -0,0 +1,27 @@
+---
+title: System Design
+description: Notes for system design interviews — how to scope a problem, size it, and pick the parts that carry the load.
+icon: network
+sidebar:
+  label: Overview
+---
+
+## How an interview runs
+
+<Steps>
+  <Step title="Requirements">
+    Pin down the functional requirements (what the system does) and the non-functional ones (latency, availability, consistency, scale) before drawing anything.
+  </Step>
+  <Step title="Estimates">
+    Size the traffic, storage and bandwidth with back-of-the-envelope numbers, so later choices have a reason.
+  </Step>
+  <Step title="API and data model">
+    Define the endpoints and the entities they read and write.
+  </Step>
+  <Step title="High-level design">
+    Draw the request path end to end: clients, load balancers, services, caches, stores and queues.
+  </Step>
+  <Step title="Deep dives">
+    Take the hardest part (a hot key, fan-out, a consistency guarantee) and work through its tradeoffs.
+  </Step>
+</Steps>