refactor(aio): move /guide/docs.md to /marketing/docs.md

Because:
1. `docs` feels like a "top level" page, similar to `features`, `resources`, `events` etc.
2. This enables the ServiceWorker to pre-fetch/cache the document (similar to what happens with all
   other direct children of `content/docs/`), without the need for special-casing it in
   `ngsw-manifest.json`.

aio/content/navigation.json

@@ -5,7 +5,7 @@
       "title": "Features"
     },
     {
-      "url": "guide/docs",
+      "url": "docs",
       "title": "Docs",
       "hidden": true
     },
@@ -21,7 +21,7 @@
 
   "SideNav": [
     {
-      "url": "guide/docs",
+      "url": "docs",
       "title": "Docs",
       "tooltip": "Angular Documentation",
       "hidden": true

aio/firebase.json

@@ -14,9 +14,9 @@
       {"type": 301, "source": "/docs/ts/latest/guide/style-guide.html", "destination": "/guide/styleguide"},
 
       // cookbook, cookbook/, cookbook/index.html
-      {"type": 301, "source": "/docs/ts/latest/cookbook",            "destination": "/guide/docs"},
-      {"type": 301, "source": "/docs/ts/latest/cookbook/",           "destination": "/guide/docs"},
-      {"type": 301, "source": "/docs/ts/latest/cookbook/index.html", "destination": "/guide/docs"},
+      {"type": 301, "source": "/docs/ts/latest/cookbook",            "destination": "/docs"},
+      {"type": 301, "source": "/docs/ts/latest/cookbook/",           "destination": "/docs"},
+      {"type": 301, "source": "/docs/ts/latest/cookbook/index.html", "destination": "/docs"},
 
       // cookbook/dependency-injection.html
       {"type": 301, "source": "/docs/ts/latest/cookbook/dependency-injection.html", "destination": "/guide/dependency-injection-in-action"},