docs: add instructions on how to extract screenshots (#4920)

docs/2-usage/12-extract-screenshots.md

@@ -0,0 +1,16 @@
+# Extract screenshots
+
+You can periodically extract screenshots from available streams by using FFmpeg inside the `runOnReady` hook:
+
+```yml
+pathDefaults:
+  runOnReady: |
+    bash -c "
+    while true; do
+      mkdir -p $(dirname screenshots/$MTX_PATH)
+      ffmpeg -i rtsp://localhost:8554/$MTX_PATH -frames:v 1 -update true -y screenshots/$MTX_PATH.jpg
+      sleep 10
+    done"
+```
+
+Where `10` is the interval between screenshots.

docs/2-usage/17-hooks.md

@@ -2,6 +2,8 @@
 
 The server allows to specify commands that are executed when a certain event happens, allowing the propagation of events to external software.
 
+## runOnConnect
+
 `runOnConnect` allows to run a command when a client connects to the server:
 
 ```yml
@@ -16,6 +18,8 @@ runOnConnect: curl http://my-custom-server/webhook?conn_type=$MTX_CONN_TYPE&conn
 runOnConnectRestart: no
 ```
 
+## runOnDisconnect
+
 `runOnDisconnect` allows to run a command when a client disconnects from the server:
 
 ```yml
@@ -24,6 +28,8 @@ runOnConnectRestart: no
 runOnDisconnect: curl http://my-custom-server/webhook?conn_type=$MTX_CONN_TYPE&conn_id=$MTX_CONN_ID
 ```
 
+## runOnInit
+
 `runOnInit` allows to run a command when a path is initialized. This can be used to publish a stream when the server is launched:
 
 ```yml
@@ -41,6 +47,8 @@ paths:
     runOnInitRestart: no
 ```
 
+## runOnDemand
+
 `runOnDemand` allows to run a command when a path is requested by a reader. This can be used to publish a stream on demand:
 
 ```yml
@@ -59,6 +67,8 @@ pathDefaults:
   runOnDemandRestart: no
 ```
 
+## runOnUnDemand
+
 `runOnUnDemand` allows to run a command when there are no readers anymore:
 
 ```yml
@@ -68,6 +78,8 @@ pathDefaults:
   runOnUnDemand:
 ```
 
+## runOnReady
+
 `runOnReady` allows to run a command when a stream is ready to be read:
 
 ```yml
@@ -88,6 +100,8 @@ pathDefaults:
   runOnReadyRestart: no
 ```
 
+## runOnNotReady
+
 `runOnNotReady` allows to run a command when a stream is not available anymore:
 
 ```yml
@@ -97,6 +111,8 @@ pathDefaults:
   runOnNotReady: curl http://my-custom-server/webhook?path=$MTX_PATH&source_type=$MTX_SOURCE_TYPE&source_id=$MTX_SOURCE_ID
 ```
 
+## runOnRead
+
 `runOnRead` allows to run a command when a client starts reading:
 
 ```yml
@@ -116,6 +132,8 @@ pathDefaults:
   runOnReadRestart: no
 ```
 
+## runOnUnread
+
 `runOnUnread` allows to run a command when a client stops reading:
 
 ```yml
@@ -125,6 +143,8 @@ pathDefaults:
   runOnUnread: curl http://my-custom-server/webhook?path=$MTX_PATH&reader_type=$MTX_READER_TYPE&reader_id=$MTX_READER_ID
 ```
 
+## runOnRecordSegmentCreate
+
 `runOnRecordSegmentCreate` allows to run a command when a recording segment is created:
 
 ```yml
@@ -139,6 +159,8 @@ pathDefaults:
   runOnRecordSegmentCreate: curl http://my-custom-server/webhook?path=$MTX_PATH&segment_path=$MTX_SEGMENT_PATH
 ```
 
+## runOnRecordSegmentComplete
+
 `runOnRecordSegmentComplete` allows to run a command when a recording segment is complete:
 
 ```yml