steam: added steam and proton quick fix

wiki/steam.md

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+# Steam
+
+[Steam](https://store.steampowered.com/) is a software for digital game
+destribution.
+
+## Proton
+
+### Errors with the Wine prefix of a specific game
+
+The last ressort for fixing problems that are related to the wine prefix of a
+game is to delete the Wine prefix.
+For this the id of the steam game first has to be known.
+This is done by navigating to the steam website or the community page of the
+specific game.
+The URLs will have one of the following forms where the game id can be read:
+
+```
+http://steamcommunity.com/app/<game id>/
+http://store.steampowered.com/app/<game id>/
+```
+
+After that navigate to the place your steam games are stored.
+You can remove `<steam location>/steamapps/compatdata/<game id>` to reset the
+Wine prefix completely.
+It is recommended to keep a backup of the folder containing the old Wine prefix
+as it stores game saves and other important data, that can be useful.