doc: add missing introduced_in metadata

Fixes doc build warnings:
```
Failed to add alternative version links to webstreams
Failed to add alternative version links to async_context
```

PR-URL: nodejs#39575
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Beth Griggs <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Darshan Sen <[email protected]>

Author: richardlau

doc/api/async_context.md

@@ -1,5 +1,7 @@
 # Asynchronous Context Tracking
 
+<!--introduced_in=v16.4.0-->
+
 > Stability: 2 - Stable
 
 <!-- source_link=lib/async_hooks.js -->

doc/api/webstreams.md

@@ -1,5 +1,7 @@
 # Web Streams API
 
+<!--introduced_in=v16.5.0-->
+
 > Stability: 1 - Experimental
 
 An implementation of the [WHATWG Streams Standard][].