Markdown pages

In a follow up of #118, I have created a Markdown version of the [standards page](http://data.okfn.org/standards).

Remarks:
* Translation of: https://github.com/okfn/data.okfn.org/blob/master/views/standards/index.
html
* The page contains 6 relational links: depending on where the page lives, they might need to be updated.
* I kept the div for the side menu `.well`, but depending on how the html page is generated (what engine is used for this?) this might need some updating
* I placed this file in the '/doc' directory. Probably a new directory should be created for all Markdown content of the site.
* This branch can be used to create markdown versions of all pages of the website (e.g. before being merged), but I'll needs some input on how best to move forward.