Based on vencax/netlify-cms-github-oauth-provider, sets up a GitHub OAuth provider for Netlify CMS authentication.
Set up serverless according to their documentation. Then install dependencies with npm install, and deploy the initial application with sls deploy. In the command line output you'll see the URL of your endpoint (something like https://{API_ID}.execute-api.us-east-1.amazonaws.com/dev/).
Because Netlify is checking the origin in OAuth requests and raw API Gateway URLs include the stage (i.e. /dev/), you'll need to associate a custom domain name with your API Gateway endpoint.
Then, create a GitHub OAuth app according to the GitHub developer documentation. The callback URL should be the API Gateway custom endpoint followed by /callback. Once you've set that up, copy the .env.yml.sample to .env.yml, and fill in the client ID and secret.
You'll need to add base_url to your CMS config file, which will be the custom API Gateway endpoint without a trailing slash.