It's where I was looking for it and couldn't find it. I kept a ref
backwards so it can still be found in the standard testing section.

The previous title sounds like it's meant for TPA for adding more TPA
runners, but this is actually about users registering their own.

Closes: team#40995

I don't believe the duplication will bring much confusion but, more
importantly, some parts of the manual
configuration (e.g. RemoveOldSignatures) are not present in the new
documentation...