minecraft: added minecraft and guide to multimc5 with optifine

wiki/minecraft.md

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+# Minecraft
+
+[Minecraft](https://www.minecraft.net) is a sandbox video game created by "Notch".
+
+## Server
+
+A server can be setup via docker with the [linuxserver image](./docker-images/itzg_-_minecraft-server.md).
+
+## Client
+
+There are different Minecraft Clients available:
+
+- The standard (Java) Minecraft Client can be downloaded at the [official website](https://minecraft.net/en-us/get-minecraft)
+- [MultiMC5](./multimc5.md) is a custom Minecraft launcher that allows multiple
+  installations at once.

wiki/multimc5.md

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+# MultiMC5
+
+[MultiMC5](https://multimc.org) is a custom launcher that allows multiple Minecraft
+instances installed at once.
+
+## Add OptiFine to a Minecraft instance
+
+[OptiFine](https://www.optifine.net/home) is a Minecraft optimization mod.
+This guide shows how to add optifine to an instance on windows.
+
+- If not already done create an instance on MultiMC5
+- Download the according version of OptiFine from their [website](https://www.optifine.net/downloads)
+- Open the Minecraft standard (Java) launcher and start the according version at
+  least once
+- Open the OptiFine jar, click `Extract` and save the extracted OptiFine to a folder
+- Edit the according instance and select `Version`, then select `Add to Minecraft.jar`
+  and select the extracted OptiFine