Add linter for the Markdown with frontmatter support

[derekherman]

Feature description

It was suggested we use the same remark-frontmatter that web.dev uses to lint the frontmatter in our Markdown files. We should also implement basic.md linting in this project.

See https://github.com/GoogleChrome/web.dev/search?q=remark-frontmatter

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

• N/A

Changelog entry

• Add linter for the Markdown with frontmatter support