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.
@nodejs/documentation @nodejs/website
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc