internal/identity: implement github provider support (#582)

Co-authored-by: Bobby DeSimone <bobbydesimone@gmail.com>

docs/.vuepress/config.js

@@ -88,6 +88,7 @@ module.exports = {
             "identity-providers/azure",
             "identity-providers/cognito",
             "identity-providers/gitlab",
+            "identity-providers/github",
             "identity-providers/google",
             "identity-providers/okta",
             "identity-providers/one-login",

docs/docs/identity-providers/github.md

@@ -0,0 +1,52 @@
+---
+title: GitHub
+lang: en-US
+# sidebarDepth: 0
+meta:
+  - name: keywords
+    content: github oauth2 provider identity-provider
+---
+
+# GitHub
+
+## Setting up GitHub OAuth2 for your Application
+
+We would like you to be aware that GitHub did not implement the OpenID Connect just OAuth2 and for this reason, we have not gotten a better way to implement revocation of user access on sign out yet.
+
+Also, the organizations a user belongs to will be used as the groups on Pomerium dashboard.
+
+Log in to [Github](https://github.com/login) or create an account.
+
+Navigate to your profile using the avatar on the navigation bar and go to your settings. 
+
+![GitHub settings](./img/github/github-user-profile.png)
+
+Click the Developers settings and create a new OAuth Application
+
+![GitHub OAuth2 Application creation](./img/github/github-oauth-creation.png)
+
+Create a new OAuth2 application by filling the field with the following parameters:
+
+Field                       | Description
+--------------------------- | --------------------------------------------
+Application name            | The name of your web app
+Homepage URL                | The homepage URL of the application to be integrated with Pomerium
+Authorization callback URL  | `https://${authenticate_service_url}/oauth2/callback`, authenticate_service_url from pomerium configuration
+
+
+After the application had been created, you will have access to the credentials, the **Client ID** and **Client Secret**.
+
+## Pomerium Configuration
+
+If the setup for GitHub OAuth application has been completed, you can create your **Pomerium** configuration like the example below:
+
+```bash
+authenticate_service_url: https://authenticate.localhost.pomerium.io
+idp_provider: "github"
+idp_client_id: "REDACTED"   // github application ID
+idp_client_secret: "REDACTED"   // github application secret
+```
+
+Whenever a user tries to access  your application integrated with Pomerium, they will be presented with a sign-on page as below:
+
+![GitHub Sign-on Page](./img/github/github-signon-page.png)