doc: add introduced_in metadata to _toc.md

Allow users to switch to the table of contents for older versions of
Node.js. This gets rid of the "Failed to add alternative version link"
warnings when building docs.

PR-URL: https://github.com/nodejs/node/pull/19113
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Matheus Marchini <matheus@sthima.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

doc/api/_toc.md

@@ -1,5 +1,8 @@
 @// NB(chrisdickinson): if you move this file, be sure to update
 @// tools/doc/html.js to point at the new location.
+
+<!--introduced_in=v0.10.0-->
+
 * [About these Docs](documentation.html)
 * [Usage & Example](synopsis.html)