doc: move configuration index to a separate page

Also add index to pipewire-props

doc/dox/config/xref.md

@@ -0,0 +1,51 @@
+\page page_config_xref Index
+
+\ref page_man_pipewire_conf_5 "pipewire.conf"
+
+@SECREF@ pipewire.conf
+
+\ref page_man_pipewire-pulse_conf_5 "pipewire-pulse.conf"
+
+@SECREF@ pipewire-pulse.conf
+
+\ref page_man_pipewire-client_conf_5 "client.conf, client-rt.conf"
+
+@SECREF@ client.conf
+
+\ref page_man_pipewire-jack_conf_5 "jack.conf"
+
+@SECREF@ jack.conf
+
+**Runtime settings**
+
+@SECREF@ pipewire-settings
+
+**Environment variables**
+
+@SECREF@ pipewire-env client-env jack-env pulse-env
+
+**Object properties**
+
+@SECREF@ props
+
+**Monitor properties**
+
+@SECREF@ monitor-prop
+
+**Device properties**
+
+@SECREF@ device-prop
+
+**Node properties**
+
+@SECREF@ node-prop
+
+**Port properties**
+
+@SECREF@ port-prop
+
+**Client properties**
+
+@SECREF@ client-prop
+
+\see pw_keys in API documentation.