Make communication channels more obvious

[sappelhoff]

fixes #306

@Remi-Gau let me know if this also fixes #291.

Also, this needs approving reviews by some, to make sure that everybody is on board with my suggestion: https://github.com/bids-standard/bids-website/issues/306#issue-1732393543

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[sappelhoff]

As discussed in yesterday's maintainers meeting, I adjusted this PR to more explicitly mention our 4 main communication channels

• google mailing list
• neurostars
• brainhack mattermost
• twitter

[sappelhoff]

I sent an email to all people who had responded to the "email newsletter survey" that we won't use these emails in the future and that they should please sign up via our 4 common communication channels.