Create a user guide for content editors

This is pulled from the docs re-org issue https://github.com/netlify/netlify-cms/issues/598#issuecomment-332564210

The current docs focus mostly on "implementers" (people writing code to configure and customize Netlify CMS on their site) and a little bit on "contributors" (people making contributions to the CMS itself). So far, they don't provide any guidance for content editors using the CMS UI to add and edit content.

I'm thinking that initially, we can put the whole guide in a single page of the docs, but I suspect we'll want to break it out into a multi-page section when we've decided how we want that to work. For now, I think getting any guidance for editors would be helpful.

Content-wise, I'm thinking this is not a place to explain what’s happening “under the hood,” though we may include links here and there for the curious. Here's a rough outline brainstorm to get anyone started who might like to take this on:

- Accessing the CMS UI: may vary, but describe common use case of website.com/admin
- troublehooting access?
- UI overview
- Sidebar
- editorial workflow - brief description, link to dedicated section
- collections
- show existing content
- create new content
- Content editor
- Accessed by selecting an existing item or creating a new one
- Brief explanation of fields and preview
- required/optional
- not all field values displayed in preview
- special widgets like image (link to media library section)
- Rich text editor
- Markdown mode
- Inserting blocks (like image)
- What “Save” does (depending on whether Editorial Workflow is enabled)
- Media library
- adding/deleting/searching for assets
- how to add to content (link to content editor)
- Editorial Workflow
- getting content into the workflow (by saving changes)
- moving centent between columns and what that means
- publishing from the ‘Waiting to go live’ column