Netlify-cms: Document "netlify-auth" backend

[mraerino]

To use the full potential of NetlifyCMS people need to know how to use the netlify-auth backend.

I was able to guess some parts from the source.

• The backend needs a auth_url and a github_proxy_url configured in config.yml
• There is no public service providing functionality for github_proxy_url
• If you want to roll your own custom authing, you need to roll a service for auth_url

Can you provide more details of how to implement those services?

[erquhart]

cc/ @biilmann, we should link to Gotiator docs here once they're published, and we can extend CMS docs to round out this specific use case.

[mryellow]

Also need to know how to use netlify-cms-github-oauth-provider... To my eyes this should be the primary use-case rather than a specific after-thought. All components with one easy setup rather than being pushed into netlify.com.

[erquhart]

@mryellow we provide Netlify's auth service as a convenience - if you're up for contributing toward a more flexible authentication path, it'd be more than welcome :)

[mryellow]

Think I found an approach where everything plays nice, something I can just throw on an instance somewhere:

https://github.com/vencax/netlify-cms-github-oauth-provider/pull/3/files

With base_url available in config things start making sense.

[tech4him1]

The netlify-auth backend was superseded by the Git Gateway backend, which is documented at https://www.netlifycms.org/docs/authentication-backends/.