Add lint rule that headings must increment one level at a time
Adds markdown rule: https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md#md001---header-levels-should-only-increment-by-one-level-at-a-time to project.
Make structure more predictable for ToC generators.
Also turns on for https://docs.gitlab.com/omnibus/README.html.