Add rule for link list indent

[koppor]

Good

[hyperref]: https://ctan.org/pkg/hyperref
[listings]: https://ctan.org/pkg/listings
[minted]: https://ctan.org/pkg/minted

Bad

  [hyperref]: https://ctan.org/pkg/hyperref
  [listings]: https://ctan.org/pkg/listings
  [minted]: https://ctan.org/pkg/minted

Explanation

I sometimes encounter Markdown documents where the list of links at the end of the document are idented by spaces. This should not happen.

I am not talking about link checking (issue #253). Meaning: Is every link listed in the block really used in the document. This would be left for real future work ^^.

This somehow refs #40