How-to Docs: Updating/syncing an existing practice exercise

Since configlet has so many options, it is sometimes hard for contributors to find the right thing to do when they want to contribute an exercise update. Would be good to have one page that can be linked in "Sync exercise" issues so a (new) contributor can easily work on such an issue.

Should include
* a link to a page that explains how to install configlet
* a note that the contributor needs to check whether the track has a generator that needs to be run or whether tests need to be updated manually

I think this docs page should still exist even if some tracks might have some automation around auto-creating PRs for parts of an exercise update. I doubt all the tracks will work that way any time soon.