Add basic docs for HTML and PDF.

Deals with #96.

Without looking in the repository, it's not clear that you can use messytables with HTML and PDFs. So, it's good to have *something* mentioning this in the docs as it helps people searching for such tools (even if this documentation perhaps need expanding upon; does @scraperdragon have anything to add?)

Haven't used Sphinx before; if there's anything amiss, please let me know!