Add docx builder extension

[Lennonka]

docxsphinx is awesome because...

• It builds Sphinx projects to Microsoft Word (docx) files, therefore promotes compatibility with a proprietary tool.
• Allows to configure a custom stylesheet in the docx or dotx format.
• The project is up to date, i.e. actively developed. However, it's possible that it doesn't support the complete set of Sphinx markup yet.
• As a technical writer with 14 years of experience, I'm using it in my work and I'd love to support further use and development.