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: #19113
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Gibson Fahnestock <[email protected]>
Reviewed-By: Matheus Marchini <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Jon Moss <[email protected]>
Reviewed-By: Sakthipriyan Vairamani <[email protected]>

Author: Trott

doc/api/_toc.md

@@ -1,5 +1,9 @@
 @// NB(chrisdickinson): if you move this file, be sure to update tools/doc/html.js to
 @// point at the new location.
+@// tools/doc/html.js to point at the new location.
+
+<!--introduced_in=v0.10.0-->
+
 * [About these Docs](documentation.html)
 * [Usage & Example](synopsis.html)