Invite links expire regularly

At the moment we're relying on invite links, which have no time limits but expire after 400 signups https://slack.com/intl/en-gb/help/articles/201330256-Invite-new-members-to-your-workspace

> On the free, Pro and Business+ subscriptions, members with permission to send invitations can also share an invitation link. Each invitation link can be used by up to 400 people.

This

1. Creates toil, because everytime the link expires, a new one has to be generated and the `slack.kedro.org` redirect needs to be updated
2. Slack doesn't tell you that a link expired, so it's up to you to monitor it, or someone complaining that the link doesn't work

I recalled there were several auto-invite services but for some reason I can't find them anymore. I did some research and it seems like Slack made some changes to the API and this cannot be easily done anymore.

An alternative could be having some sort of "ping" that checks whether the link still works to send an automated alert. But it's still very reactive.

Another solution could be tracking the number of new users that have joined since the last link was enacted. That way we can preemptively generate a new one when it's about to expire.

The issue names no repository files, tests, or implementation entry points. Start by evaluating the proposed Slack invite-link monitoring and signup-count approaches, then clarify the preferred scope. Done means an agreed solution can detect or prevent link expiry without relying on manual redirect updates.

Written by the indexing model from the issue text.