Point corporate donors toward our OC contact form

[colindean]

A corporate donor recently wasn't able to find a clear way to contact a Homebrew person to talk about a possible donation. They eventually found the OC contact us page, which is the best way to contact the PLC short without having a GitHub account and posting on an issue or Discussions.

Let's highlight the donations questions contact form specifically in this area that is the landing target for the donations link on https://brew.sh.

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• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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[MikeMcQuaid]

Thanks @colindean!