doc: update vm.md for link linting

Update vm.md for link linting rules that are about to be applied. Define
an undefined link reference. Escape non-link text that uses `[` and `]`.

PR-URL: https://github.com/nodejs/node/pull/29982


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>