fzf: add script for finding 'share' folder

So that helper scripts can be easily sourced in interactive shell
configuration. `autojump` package was already present and had the same
requirements for findind a `share` folders, so I took an inspiration
there.

I beleive this is a better alternative to:
- https://github.com/NixOS/nixpkgs/pull/25080
- https://github.com/NixOS/nixpkgs/pull/27058

Replacing `$out/share/shell` with `$bin/share/fzf` was necessary to
prevent dependency loop in produced derivations.

doc/package-notes.xml

@@ -366,15 +366,33 @@ it. Place the resulting <filename>package.nix</filename> file into
 
 </section>
 
-<section xml:id="sec-autojump">
+<section xml:id="sec-shell-helpers">
 
-<title>Autojump</title>
+<title>Interactive shell helpers</title>
 
 <para>
-  autojump needs the shell integration to be useful but unlike other systems,
-  nix doesn't have a standard share directory location. This is why a
-  <command>autojump-share</command> script is shipped that prints the location
-  of the shared folder. This can then be used in the .bashrc like this:
+  Some packages provide the shell integration to be more useful. But
+  unlike other systems, nix doesn't have a standard share directory
+  location. This is why a bunch <command>PACKAGE-share</command>
+  scripts are shipped that print the location of the corresponding
+  shared folder.
+
+  Current list of such packages is as following:
+
+  <itemizedlist>
+    <listitem>
+      <para>
+        <literal>autojump</literal>: <command>autojump-share</command>
+      </para>
+    </listitem>
+    <listitem>
+      <para>
+        <literal>fzf</literal>: <command>fzf-share</command>
+      </para>
+    </listitem>
+  </itemizedlist>
+
+  E.g. <literal>autojump</literal> can then used in the .bashrc like this:
 <screen>
   source "$(autojump-share)/autojump.bash"
 </screen>