Add development section

* I cretaed a folder 'development'
* I split up the file dev.md into three parts and moved it to this folder
  * index.md
  * authentication_authorization.md
  * mrf.md
* I also moved ap_extensions.md
* I created a new file setting_up_pleroma_dev.md

docs/development/API/prometheus.md

@@ -0,0 +1,44 @@
+# Prometheus Metrics
+
+Pleroma includes support for exporting metrics via the [prometheus_ex](https://github.com/deadtrickster/prometheus.ex) library.
+
+Config example:
+
+```
+config :prometheus, Pleroma.Web.Endpoint.MetricsExporter,
+  enabled: true,
+  auth: {:basic, "myusername", "mypassword"},
+  ip_whitelist: ["127.0.0.1"],
+  path: "/api/pleroma/app_metrics",
+  format: :text
+```
+
+* `enabled` (Pleroma extension) enables the endpoint
+* `ip_whitelist` (Pleroma extension) could be used to restrict access only to specified IPs
+* `auth` sets the authentication (`false` for no auth; configurable to HTTP Basic Auth, see [prometheus-plugs](https://github.com/deadtrickster/prometheus-plugs#exporting) documentation)
+* `format` sets the output format (`:text` or `:protobuf`)
+* `path` sets the path to app metrics page 
+
+
+## `/api/pleroma/app_metrics`
+
+### Exports Prometheus application metrics
+
+* Method: `GET`
+* Authentication: not required by default (see configuration options above)
+* Params: none
+* Response: text
+
+## Grafana
+
+### Config example
+
+The following is a config example to use with [Grafana](https://grafana.com)
+
+```
+  - job_name: 'beam'
+    metrics_path: /api/pleroma/app_metrics
+    scheme: https
+    static_configs:
+    - targets: ['pleroma.soykaf.com']
+```