split out callbacks to its own file

SUMMARY.md

@@ -52,6 +52,7 @@
 * [How to HTTP with curl](http.md)
 * [Building and installing](building.md)
 * [libcurl basics](libcurl.md)
+    * [Callbacks](libcurl-callbacks.md)
     * [API compatibility](libcurl-api.md)
     * [--libcurl](libcurl--libcurl.md)
     * [Header files](libcurl-headers.md)

libcurl-callbacks.md

@@ -0,0 +1,64 @@
+## Callbacks
+
+TBD
+
+### Write callback
+
+TBD
+
+### Read callback
+
+TBD
+
+### Progress callbacks
+
+TBD
+
+### Header callback
+
+TBD
+
+### Debug callback
+
+TBD
+
+### SSL context callback
+
+TBD
+
+### ioctl and seek callbacks
+
+TBD
+
+### Convert to and from network callbacks
+
+TBD
+
+### Convert from UTF8 callback
+
+TBD
+
+### Sockopt callback
+
+TBD
+
+### Opensocket and closesocket callbacks
+
+TBD
+
+### SSH key callback
+
+TBD
+
+### RTSP interleave callback
+
+TBD
+
+### FTP chunk callbacks
+
+TBD
+
+### FTP matching callback
+
+TBD
+

libcurl.md

@@ -96,70 +96,6 @@ TBD
 
 TBD
 
-## Callbacks
-
-TBD
-
-### Write callback
-
-TBD
-
-### Read callback
-
-TBD
-
-### Progress callbacks
-
-TBD
-
-### Header callback
-
-TBD
-
-### Debug callback
-
-TBD
-
-### SSL context callback
-
-TBD
-
-### ioctl and seek callbacks
-
-TBD
-
-### Convert to and from network callbacks
-
-TBD
-
-### Convert from UTF8 callback
-
-TBD
-
-### Sockopt callback
-
-TBD
-
-### Opensocket and closesocket callbacks
-
-TBD
-
-### SSH key callback
-
-TBD
-
-### RTSP interleave callback
-
-TBD
-
-### FTP chunk callbacks
-
-TBD
-
-### FTP matching callback
-
-TBD
-
 ## Easy API
 
 TBD