docs: fill all TBD documentation pages and add new harness module pages

Agent-Logs-Url: https://github.com/bytedance/deer-flow/sessions/ff389ed8-31c9-430c-85ff-cc1b52b8239c

Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>

frontend/src/content/en/harness/_meta.ts

@@ -10,6 +10,12 @@ const meta: MetaRecord = {
   "design-principles": {
     title: "Design Principles",
   },
+  "lead-agent": {
+    title: "Lead Agent",
+  },
+  middlewares: {
+    title: "Middlewares",
+  },
   configuration: {
     title: "Configuration",
   },
@@ -25,6 +31,12 @@ const meta: MetaRecord = {
   sandbox: {
     title: "Sandbox",
   },
+  subagents: {
+    title: "Subagents",
+  },
+  mcp: {
+    title: "MCP Integration",
+  },
   customization: {
     title: "Customization",
   },