docs: use invite link for discord

[booniepepper]

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The previous link to the #aqua channel in /dev/tty Discord server will only work for users that have already been invited to the server.

This invite link will still take people to the #aqua channel.

[suzuki-shunsuke]

📝

I see. I'm not familiar with Discord, but I've confirmed

• I can create an invitation link myself
• We can create an invitation link without expiration date

[suzuki-shunsuke]

Thank you always!