Add documentation for ENVied & SiteConfig (#5841)

* Add documentation for ENVied & SiteConfig

* Add images

* Fix swapped screenshots and update list

docs/backend/configuration.md

@@ -0,0 +1,42 @@
+---
+title: Configuration
+---
+
+# Configuration
+
+We currently use two gems for configuring the application:
+
+- [ENVied](https://github.com/eval/envied)
+- [rails-settings-cached](https://github.com/huacnlee/rails-settings-cached)
+
+## ENVied
+
+This gem is primarily used for configuring environment variables related to
+credentials and third party services. Examples:
+
+- `REDIS_URL`
+- `FASTLY_API_KEY`
+- `STRIPE_SECRET_KEY`
+
+Settings managed via ENVied can be found in
+[`Envfile`](https://github.com/thepracticaldev/dev.to/blob/master/Envfile) (see
+[Configuring Environment Variables](../getting-started/config-env.md)) and
+viewed at `/internal/config` (see [Accessing the admin panel](./admin.md)):
+
+![Screenshot of env variable admin interface](https://user-images.githubusercontent.com/47985/73627243-67d41f80-467e-11ea-9121-221275ff8a89.png)
+
+## rails-settings-cached
+
+We use this gem for managing settings used within the app's business logic.
+Examples:
+
+- `main_social_image`
+- `rate_limit_follow_count_daily`
+- `suggested_tags`
+
+These settings can be accessed via the
+[`SiteConfig`](https://github.com/thepracticaldev/dev.to/blob/master/app/models/site_config.rb)
+object and viewed / modified via `/internal/config` (see
+[Accessing the admin panel](./admin.md)).
+
+![Screenshot of site configuration admin interface](https://user-images.githubusercontent.com/47985/73627238-6276d500-467e-11ea-8724-afb703f056bc.png)

docs/backend/readme.md

@@ -5,6 +5,7 @@ items:
   - auth-twitter.md
   - auth-github.md
   - authorization.md
+  - configuration.md
   - roles.md
   - algolia.md
   - pusher.md