Document how to work with Netlify CMS

[mazipan]

Description

Explain how to gain credentials to the Netlify CMS dashboard, how to update/add some configuration, and how to create new content to Netlify CMS.

Tasks

• Document how to gain credentials to Netlify CMS
• Document how to update/add Netlify CMS configuration items
• Document how to create/update content in Netlify CMS
• Document how the Netlify CMS editorial workflow works

[resir014]

@mazipan We should also add some documentations on how to gain credentials to our content dashboards (e.g. Netlify CMS and Cloudinary), so that no further confusion arises like in #696.

For the time being, as mentioned here by @zainfathoni, the only way to gain these credentials is to ask either of us. And do take note that for security reasons, we only provide these credentials to those who, really, really, really need it.