doc: enable first-heading-level remark-lint rule

This makes sure that the first heading in all markdown files is h1.

Only three files needed to be changed to conform:
 1. BUILDING.md
 2. CODE_OF_CONDUCT.md
 3. doc/onboarding-extras.md

Also, .remarkrc is updated to include the `first-heading-level: 1`
rule in order to catch similar issues in the future.

PR-URL: https://github.com/nodejs/node/pull/8716


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>