docs: split and expand backend docs (#3350)

website/content/docs/github-backend.md

@@ -0,0 +1,34 @@
+---
+title: GitHub
+weight: 20
+group: backends
+---
+
+For repositories stored on GitHub, the `github` backend allows CMS users to log in directly with their GitHub account. Note that all users must have push access to your content repository for this to work.
+
+Because Github [requires a server](https://github.com/netlify/netlify-cms/issues/663#issuecomment-335023723) for authentication, Netlify facilitates basic GitHub authentication.
+
+To enable basic GitHub authentication:
+
+1. Follow the authentication provider setup steps in the [Netlify docs](https://www.netlify.com/docs/authentication-providers/#using-an-authentication-provider).
+2. Add the following lines to your Netlify CMS `config.yml` file:
+  ```yaml
+  backend:
+    name: github
+    repo: owner-name/repo-name # Path to your GitHub repository
+  ```
+
+## Specifying a status for deploy previews
+The GitHub backend supports [deploy preview links](../deploy-preview-links). Netlify CMS checks the
+`context` of a commit's [statuses](https://help.github.com/articles/about-status-checks/) and infers
+one that seems to represent a deploy preview. If you need to customize this behavior, you can
+specify which context to look for using `preview_context`:
+
+```yaml
+backend:
+  name: github
+  repo: my/repo
+  preview_context: my-provider/deployment
+```
+
+The above configuration would look for the status who's `"context"` is `"my-provider/deployment"`.