Add lint check for multiple newlines.

[alexfornuto]

It might be good to enable this plugin in our remark config so we can keep things this way: https://github.com/remarkjs/remark-lint/tree/main/packages/remark-lint-no-consecutive-blank-lines

Originally posted by @ptgott in gravitational/teleport#24247 (comment)

[ptgott]

Closing since we have removed formatting linters that don't affect the rendered page. See #508