Add a Configuration doc

[verythorough]

Configuration is covered briefly in the Quick Start, and even more briefly in the Intro, but really needs a dedicated page, providing a reference for all of the possible fields and settings for config.yml. Here's a basic outline taken from Issue #598:

Configuration options (config.yml)

• backend: explanation, available options, link to Authentication doc (track Add instructions for auth with Netlify Identity and Git Gateway #600)
• publish_mode: how editorial workflow works with Git, link to content editor UI docs
• media_folder: what this means, what’s a valid path
• public_folder: relationship to media_folder, default, valid path info
• collections: each named collection displays in the sidebar of the app
  • name
  • label: note that this not pluralized (or is it? see Add label_plural field to collection config #684)
  • folder or file: briefly explain two types, link to dedicated doc (track Add Collection Types doc #679)
  • filter: explain, link to doc (track Add Collection Types doc #679)
  • create: valid with folder type only (?)
  • slug: template options, default (?)
  • fields: will generate key value pairs in data files, or in the frontmatter of markdown files
    • name: note special body value for the main content section of md files
    • label
    • widget: explain, link to dedicated doc (track Widget docs a bit thin #619)
    • default
    • required: defaults to true
    • pattern: basic validation described in validation.md (eventually removing this doc after copying the contents to this and the custom widgets doc (track Widget docs a bit thin #619)

Note that in several sections, the doc refers to other, canonical docs, rather than repeating a full reference.