Create XML style sheet to display SDF

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/134)) by Nate Koenig (Bitbucket: [Nathan Koenig](https://bitbucket.org/%7Bc862cdd9-fcc8-4419-9fc7-e20db14b8fcb%7D/)).**

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Similar to the [Google style guide](http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml), create an XML style sheet to display documented SDF. This would avoid the need to compile and run `gzsdf` to produce docs for SDF. Replaces #27.

Moved from [gazebo issue #226](https://bitbucket.org/osrf/gazebo/issues/226).

Start by reviewing the archived report, the linked Google XML style guide, and how gzsdf currently produces SDF documentation. Define the stylesheet's expected input and output before implementing it. Done means documented SDF can be displayed through the stylesheet without compiling and running gzsdf.

Written by the indexing model from the issue text.