Move core user docs back to appropriate sections

Requested by: https://github.com/QubesOS/qubes-doc/pull/823#issuecomment-496316322
Related to: QubesOS/qubes-issues#4693

user/advanced-configuration/i3.md

@@ -0,0 +1,90 @@
+---
+layout: doc
+title: i3
+permalink: /doc/i3/
+redirect_from:
+- /en/doc/i3/
+- /doc/i3/
+- "/doc/UserDoc/i3/"
+- "/wiki/UserDoc/i3/"
+---
+
+# i3 installation in dom0
+
+i3 is part of the stable repository (as of Qubes R3.1) and can be installed by
+using the [dom0 update mechanism](/doc/software-update-dom0/). To install the i3
+window manager and the its Qubes specific configuration:
+
+    $ sudo qubes-dom0-update i3 i3-settings-qubes
+
+The Qubes-specific configuration (package `i3-settings-qubes`) can be installed
+optionally in case you would prefer writing your own configuration (see
+[customization](#customization) section for scripts and configuration).
+
+That's it. After logging out, you can select i3 in the login manager.
+
+### Customization
+
+* [xdg_autostart_script](https://gist.github.com/SietsevanderMolen/7b4cc32ce7b4884513b0a639540e454f)
+* [i3bar_script](https://gist.github.com/SietsevanderMolen/e7f594f209dfaa3596907e427b657e30)
+* [terminal_start_script](https://gist.github.com/SietsevanderMolen/7c6f2b5773dbc0c08e1509e49abd1e96)
+* [i3 config with dmenu-i3-window-jumper](https://github.com/anadahz/qubes-i3-config/blob/master/config)
+
+## Compilation and installation from source
+
+Note that the compilation from source is done in a Fedora based domU (could
+be dispvm). The end result is always an `.rpm` that is copied to dom0 and then
+installed through the package manager.
+
+### Getting the code
+
+Clone the i3-qubes repository here:
+
+    $ git clone https://github.com/QubesOS/qubes-desktop-linux-i3
+
+In this case, the most interesting file is probably
+`i3/0001-Show-qubes-domain-in-non-optional-colored-borders.patch` It's the patch
+with changes that are necessary to make i3 work nicely with Qubes OS. The code
+should not need much explanation, it just gets the vmname and label from Qubes
+OS and changes some defaults so the user can't override decisions.
+
+If you want to make any changes to the package, this is the time and place to do
+it.
+
+### Building
+
+You'll need to install the build dependencies, which are listed in
+build-deps.list. You can verify them and then install them with:
+
+    $ sudo dnf install -y $(cat build-deps.list)
+
+This used to be more complicated, but I finally redid this and use the same
+buildsystem that's used by Qubes OS for XFCE. It's just a Makefile that helps
+you get the sources and start off the build:
+
+    $ make rpms
+
+### Installing
+
+**Warning**: Manually installing software in dom0 is inherently risky, and the method described here circumvents the usual security mechanisms of qubes-dom0-update.
+
+You should now have your i3 rpm in `./rpm/x86_64/i3-4.8-3.fc20.x86_64.rpm`.
+Protip: copying this file to `~/i3.rpm` now will save you some typing in the
+next step.
+
+Now in dom0, copy in the rpm:
+
+    $ qvm-run --pass-io <src_domain> 'cat </path/to/rpm_in_src_domain>' > i3.rpm
+
+Now that the rpm is in dom0 we can proceed with installing it. i3 has some
+dependencies that we can easily install with:
+
+    $ sudo qubes-dom0-update perl-AnyEvent-I3 xorg-x11-apps \\
+        rxvt-unicode xcb-util-wm perl-JSON-XS xcb-util-cursor \\
+        dzen2 dmenu xorg-x11-fonts-misc libev
+
+After that you can just install the generated rpm like any other local package:
+
+    $ sudo yum localinstall i3.rpm
+
+Log out, select i3, then log in again.