docs: Flesh out uninstallation documentation

[ci skip]

docs/getting-started/uninstalling.md

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+# Uninstalling
+
+While we hate to see you go, if you need to uninstall Dokku, the following may help you out:
+
+## Arch Installation
+
+```shell
+# purge dokku from your system
+yaourt -Rsn dokku
+```
+
+## CentOS Installation
+
+
+```shell
+# uninstall dokku
+yum remove dokku herokuish
+```
+
+## Debian Installation
+
+```shell
+# purge dokku from your system
+apt-get purge -y --force-yes dokku herokuish
+
+# remove any dependencies that are no longer necessary
+apt-get -y --force-yes autoremove
+```
+
+## Makefile Installation
+
+This is a manual deletion process, and as it is not a recommended installation method, there is currently no automated uninstallation.
+
+All service plugins should be unlinked from applications, stopped, and destroyed.
+
+All applications should be stopped, and all docker containers and images deleted:
+
+```shell
+# stop all applications
+dokku --quiet apps | xargs dokku ps:stop
+
+# cleanup containers and images
+dokku cleanup
+```
+
+The following user/group must be deleted:
+
+- user: `dokku`
+- group: `dokku`
+
+The following directories must be deleted:
+
+- `~dokku`
+- `/var/lib/dokku`
+- `/var/log/dokku`