watchdog: Separate kind of documentation

Currently there are several (sub-)documents for "Generic kernel
infrastructure API" and several "driver specific" documents. Put each
one into its own sub-section.

Signed-off-by: Philipp Hahn <phahn-oss@avm.de>
Link: https://lore.kernel.org/r/7d1b722205bab83603832e66750f7b5f1f73eaa5.1777972790.git.phahn-oss@avm.de
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/watchdog/index.rst

@@ -4,15 +4,25 @@
 Watchdog Support
 ================
 
+Kernel infrastructure
+=====================
+
 .. toctree::
     :maxdepth: 1
 
-    hpwdt
-    mlx-wdt
-    pcwd-watchdog
     watchdog-api
     watchdog-kernel-api
-    watchdog-parameters
     watchdog-pm
-    wdt
     convert_drivers_to_kernel_api
+
+Driver specific
+===============
+
+.. toctree::
+    :maxdepth: 1
+
+    watchdog-parameters
+    hpwdt
+    wdt
+    mlx-wdt
+    pcwd-watchdog