doc: Adds xml fixing script. (see previous and next commits)

This script is used to automatically fix issues within xml documentation
files.

The script is *for now* intended to be used ad-hoc, and the commits to
be examined.

A future discussion will define whether:

  * This commit and scripts are kept.
  * The script is extended for common use.

The biggest issue right now with the script is that it *could* in theory
destroy a valid space-less varlistentry.

The script could, in practical use, be changed and extended to normalize
some parts of the XML files, mainly:

  * A common quoting style for attributes
  * Fix-up some weird formatting automatically that xmlformat doesn't
    catch

doc/shell.nix

@@ -1,5 +1,5 @@
 { pkgs ? import ../. {} }:
 (import ./default.nix).overrideAttrs (x: {
-  buildInputs = x.buildInputs ++ [ pkgs.xmloscopy ];
+  buildInputs = x.buildInputs ++ [ pkgs.xmloscopy pkgs.ruby ];
 
 })