qosmngr: Add documentation for dscp2pbit mapping configuration

docs/guide/dscp2pbit.md

+## About DSCP-to-Pbit mapping
+
+DSCP-to-Pbit mapping refers to the process of associating Differentiated Services Code Point (DSCP)
+values with Priority Bits (Pbits) in a network.
+
+## Mapping Rule
+
+A mapping rule defines how specific DSCP values are associated with Pbits.
+For example:
+
+If DSCP value is 2, the mapped Pbit is 3.
+For DSCP values 23, 24, 25, 26, 27, the mapped Pbit is 1.
+
+## Configuration of mapping rule
+
+The mapping rule can be configured as a generic rule system wide, but it is
+a better strategy to have it configured on ingress traffic per port basis.
+In this case there is one config section per mapping rule per port.
+
+For example:
+To configure all 64 DSCP-to-Pbit mapping on port eth0.2 the following config
+can be used:
+
+```bash
+config classify 'c1'
+    option order '1 '
+    option enable '1'
+    option ifname 'eth0.2'
+    option dscp_filter '0'
+    option pcp_mark '2'
+
+config classify 'c2'
+    option order '2 '
+    option enable '1'
+    option ifname 'eth0.2'
+    option dscp_filter '1'
+    option pcp_mark '2'
+
+config classify 'c3'
+    option order '3'
+    option enable '1'
+    option ifname 'eth0.2'
+    option dscp_filter '2'
+    option pcp_mark '3'
+
+config classify 'c4'
+    option order '4'
+    option enable '1'
+    option ifname 'eth0.2'
+    option dscp_filter '3'
+    option pcp_mark '3'
+
+config classify 'c5'
+    option order '5'
+    option enable '1'
+    option ifname 'eth0.2'
+    option dscp_filter '4'
+    option pcp_mark '4'
+
+config classify 'c6'
+    option order '6'
+    option enable '1'
+    option ifname 'eth0.2'
+    option dscp_filter '5'
+    option pcp_mark '4'
+.
+.
+.
+config classify 'c64'
+    option order '64'
+    option enable '1'
+    option ifname 'eth0.2'
+    option dscp_filter '63'
+    option pcp_mark '7'
+```
+
+Adding mapping rule via TR181:
+- Add Device.QoS.Classification. object.
+- Set port name, Device.QoS.Classification.{i}.Interface
+Note: generally its a reference to Device.Ethernet.Interface.{i}. object
+- Set DSCP classification criteria, Device.QoS.Classification.{i}.DSCPCheck
+- Set pbit marking value, Device.QoS.Classification.{i}.EthernetPriorityMark
+- Finally enable the config, set Device.QoS.Classification.{i}.Enable to true
+
+### Example data model configuration
+
+```bash
+obuspa -c add Device.QoS.Classification.
+obuspa -c set Device.QoS.Classification.1.Interface Device.Ethernet.Interface.3.
+obuspa -c set Device.QoS.Classification.1.DSCPCheck 2
+obuspa -c set Device.QoS.Classification.1.EthernetPriorityMark 3
+obuspa -c set Device.QoS.Classification.1.Enable true
+```
+
+### A UCI mapping rule config looks as follows:
+
+```
+config classify 'classify_1'
+	option enable '1'
+	option order '1'
+	option ifname 'eth0.3'
+	option pcp_mark '3'
+	option dscp_filter '2'
+```
+
+Note:
+- Multiple config can be added this way to create mapping rule for all 64 DSCP values.
+- The configured mapping rule can be viewed in the ebtables 'broute' table's 'dscp2pbit' chain