Add documentation about grub and cleanup

docs/Cleanup.md

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+# Cleanup
+
+This document refers to information about cleaning the system, such as deleting
+old generations, unused nix shells, etc.
+
+## Delete Old Generations
+
+Run the following commands to clean the system up and delete old generations:
+
+```bash
+nix-collect-garbage --delete-old
+sudo nox-collect-garbage -d
+sudo /run/current-system/bin/switch-to-configuration boot
+```

docs/Grub.md

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+# Grub
+
+This document is for information regarding Grub.
+
+## Reinstall Grub
+
+If the grub install breaks and you ever need to reinstall grub, the following
+command is how you do it:
+
+```bash
+sudo NIXOS_INSTALL_BOOTLOADER=1 /nix/var/nix/profiles/system/bin/switch-to-configuration boot
+```