Update manual install instructions to mention deleting files first

[GeckoEidechse]

Sooner or later there will be a case where we have to move script files around or rename them.

With the current manual install instructions suggesting to just override, that means that we cannot do that without causing errors for some players.

As such while there was no need to move files yet, we should already prepare for such a case by updating the instructions to say to delete Northstar.Custom, Northstar.Client, and Northstar.CustomServers before before overriding files.

Mod-managers do not delete files either yet before updating but that can easily be changed and propagates faster than a change to docs. So updating docs is the place to start ^^