Move /api to /docs/api

Also make API docs responsive so they can be viewed
on mobile.

REST docs are at /docs/api/rest and WebSocket docs
are at /docs/api/websocket.

frontend/src/app/components/docs/docs.component.ts

@@ -0,0 +1,21 @@
+import { Component, OnInit } from '@angular/core';
+import { ActivatedRoute } from '@angular/router';
+
+@Component({
+  selector: 'app-docs',
+  templateUrl: './docs.component.html',
+  styleUrls: ['./docs.component.scss']
+})
+export class DocsComponent implements OnInit {
+
+  activeTab = 0;
+
+  constructor(
+    private route: ActivatedRoute,
+  ) { }
+
+  ngOnInit(): void {
+    const url = this.route.snapshot.url;
+    this.activeTab = ( url[2].path === "rest" ) ? 0 : 1;
+  }
+}