Add vale rule to warn about EOL whitespace

What does this MR do?

The TW team has to clean up stray whitespace every month, so we should have a vale rule to gently warn people when they are introducing EOL whitespace. This new vale rule does that, at the warning level so that it doesn't fail pipeline.

Now that more people have the docs tooling installed in their editors, I'm hopeful this will reduce how much whitespace slips in each month.

I verified every warning flagged by this new rule and found no false positives, so I think this is working as intended.

Related issues

Related to technical-writing#607 (closed)

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