📝 Documentation: Mention Subscribe button on comment-meaningless docs page

[JoshuaKGoldberg]

Documentation Report Checklist

• I have pulled the latest main branch of the repository.
• I have searched for related issues and found none that matched my issue.

Overview

As mentioned by @rviscomi in https://bsky.app/profile/rviscomi.dev/post/3lnuzxa23fk2x:

Sometimes people leave a comment as a way of subscribing for updates and don't realize there's a button for that

I didn't think to mention that in octo.guide/rules/comment-meaningless, but that's a great point - I forgot people do that. Accepting docs PRs.

Additional Info

Co-authored-by: @rviscomi

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