tools: lint for additional strings in docs

Check for uses of `Github` (rather than `GitHub`) and `Node.JS` (rather
than `Node.js`) in markdown files.

PR-URL: https://github.com/nodejs/node/pull/17492


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>