A directory-category for terminal emulators

This is a mostly cosmetical commit, in the sense it doesn't change the contents
of any package, but reorganizes the overall Nixpkgs expressions.

Terminal emulators are an ubiquitous tool for any Unix user; even the beginners
are routinely familiarized to it. And, manifestly, there are many
implementations of terminal emulators out there, from those traditionally made
in C and C++ to those written in Haskell and Go.

Terminal emulators deserve more highlight. This commit does that by creating a
category for them.

doc/contributing/coding-conventions.xml

@@ -522,6 +522,16 @@ args.stdenv.mkDerivation (args // {
          </para>
         </listitem>
        </varlistentry>
+       <varlistentry>
+        <term>
+         If it’s a <emphasis>terminal emulator</emphasis>:
+        </term>
+        <listitem>
+         <para>
+          <filename>applications/terminal-emulators</filename> (e.g. <filename>alacritty</filename> or <filename>rxvt</filename> or <filename>termite</filename>)
+         </para>
+        </listitem>
+       </varlistentry>
        <varlistentry>
         <term>
          If it’s for <emphasis>video playback / editing</emphasis>: