users: Troubleshooting for "folder marker missing" error messages

[GermanCoding]

Primary motivations are many forum threads over the years such as this one: https://forum.syncthing.net/t/the-problem-with-forum-support/20154

Also an old discussion: syncthing/syncthing#5207

Once/If this PR is merged, I would also like to link it from within the error message in syncthing, so that it can be found immediatly.

[calmh]

Great. This might even deserve its own page, and we can iterate, but this a very good start.

[GermanCoding]

So how do we want to go about this? I would like to link to the troubleshooting guide from within the error message. This currently says:

folder marker missing (this indicates potential data loss, search docs/forum to get information about how to proceed)

This message could be made more clear by changing it to include some sort of link to the docs guide. However, if we want to move this to a separate page we should do so first. Otherwise maintaining link because otherwise maintaining link availability annoying.

I can look into merging the current folder FAQ (which looks a bit split currently) into its own page, if that's what we want.

[calmh]

This is from memory as I'm not at a real computer, but I think we have the capability to handle arbitrary redirects on the doc site. So we could point towards something generic like /users/folder-marker-missing and handle that with a redirect to the FAQ entry or in the future to a separate page. (Essentially guessing the URL to the future page today.)