Add trailing spaces check to lefthook

What does this MR do?

From discussion in: !91423 (merged), this is one way to minimize trailing whitespace being introduced into the documentation suite. Only applies to folks who've opted-in to Lefthook (and are, therefore, accustomed to pushes failing because of checks) and only for Markdown in /doc.

To test:

1. Check out branch.
2. Introduce a trailing whitespace error and commit the change.
3. Run bundle exec lefthook run pre-push.

@kpaizee @dianalogan

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