[LINTER] Accept hard line breaks in markdown files

Summary:
D7483 showed that the trailing whitespaces can actually be a feature from the markdown syntax, so they need to be excluded from the linter.
Exactly 2 whitespace is a [[ https://github.github.com/gfm/#hard-line-breaks | hard break ]]with the github flavored markdown (and maybe others).

Test Plan:
No issue:

arc lint --everything

Add trailing a single or 3+ spaces to some markdown files, then:

arc lint

Ensure there is no error reported.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7484