docs: lint JS snippets in Markdown and HTML files too

[XhmikosR]

No description provided.

[rohit2sharma95]

There is an eslint plugin for markdown files though:
https://github.com/eslint/eslint-plugin-markdown

But it only lints fenced code blocks in markdown files.

[XhmikosR]

That's what I had in mind too :)

Feel free to make a PR if you get it to work; linting md files is better than nothing and now we do use fenced code blocks for most of the things apart from examples.