Move link fixing of this repository's content to a tech support activity

[StevenMaude]

It's useful to flag and fix broken links because:

• it ensures that readers don't have to search for the correct link themselves
• it may highlight that surrounding content is now out-of-date and requires updating

Currently, I tend to fix the links in this repository as and when I get round to it. The notifications go to the #team-rex Slack channel.

It's preferable for this task to be a tech support duty, owned by the whole tech team, so that it happens at least semi-regularly. (The other option here is that it's a regular duty for someone in the researcher experience team.)

Handing over this task will require:

• sending notifications of link check failures in a Slack channel such as #tech-support
• documenting the process for fixing links (already described in the documentation's README)
• informing everyone on tech support that this is a regular duty, adding information to the tech support playbook
• reviewing after some period of time to see whether or not this has been successful