docs: Add Debugging long mutagen startup time

[rfay]

The Issue

At Drupalcon Portland 2024 someone was explaining how much they liked mutagen on their macOS amd64 machine, but their project could take 30 minutes to do the syncing.

I wasn't able to figure out working with them what the problem was. I eventually determined that we used to show the files syncing as they happened, but now we've replace it with dots, which makes debugging much harder.

How This PR Solves The Issue

Provide a technique for debugging.

Manual Testing Instructions

Follow the instructions. To create a dummy file if you need one, use dd if=/dev/zero of=dummyfile bs=1M count=5000

Release/Deployment Notes

[rfay]

When mutagen is enabled we still say For details on sync status 'ddev mutagen st wind10 -l' as if that were useful. It shows errors if there are any, but it doesn't repeat., so you can't see the kind of issues considered in this PR.

It's possible we should give the full command here instead of showing that.