Move "contributing guide" to be published on the website

[madcampos]

Right now the contributing guide is a set of markdown docs living on the docs folder on github.

We should expand them and add them as a series of blog posts to the blog itself (be very meta about it).

We want to split the posts in two parts: one for content writing and another for code contributions.

One reference to use is here: https://www.a11yproject.com/write-for-us/