tools: lint doc code examples in strict mode

PR-URL: https://github.com/nodejs/node/pull/21615
Refs: https://github.com/eslint/eslint-plugin-markdown#strict


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>