qosmngr: Add documentation for VLAN and Pbit translation config

docs/guide/VLAN_Pbit_translation.md

+# VLAN and Pbit Translation
+## About VLAN Translation:
+VLAN translation, also known as VLAN mapping, is a technique used to associate different VLAN tags between customer networks and service provider networks. It involves swapping an incoming VLAN tag with a new VLAN tag.
+## Scenario
+Imagine a situation where customers and service providers use different VLAN tags in their networks.
+The same VLAN tag for a customer may correspond to a different VLAN tag in the service provider’s network.
+For example:
+C-VLAN: Customer VLAN (used by the customer network): VLAN 100
+S-VLAN: Service provider VLAN (used by the service provider network): VLAN 200
+
+The VLAN translation should happen both ways in order to ensures smooth communication across different VLANs in heterogeneous network environments
+## Configuration of VLAN translatinon
+VLAN translation can be done on the ingress traffic on the basis of following classification criteria:
+- Port name
+- Source/destination MAC address
+- VLAN ID
+- VLAN Pbit value
+### 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 VLAN classification criteria ID, Device.QoS.Classification.{i}.VLANIDCheck
+- Set VALN translation ID, Device.QoS.Classification.{i}.X_IOPSYS_EU_VLANIDMark
+- Finally enable the config, set Device.QoS.Classification.{i}.Enable to true
+- Now, repeat the above step for reverse translation also
+### 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.VLANIDCheck 100
+obuspa -c set Device.QoS.Classification.1.X_IOPSYS_EU_VLANIDMark 200
+obuspa -c set Device.QoS.Classification.1.Enable true
+
+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.VLANIDCheck 200
+obuspa -c set Device.QoS.Classification.1.X_IOPSYS_EU_VLANIDMark 100
+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 vid_check '100'
+	option vid_mark '200'
+	
+config classify 'classify_2'
+	option enable '1'
+	option order '1'
+	option ifname 'eth0.3'
+	option vid_check '200'
+	option vid_mark '100'
+```
+## About Pbit/802.1p Translation:
+802.1p is a quality of service (QoS)/class of service (CoS) method that operates at the MAC layer (Layer 2). Equipment that supports 802.1p can add and recognize a value that indicates the priority level of the Ethernet frame.
+## Scenario
+Suppose we need to remark the Pbit value coming from one of the LAN port.
+Pbit 3 should be remarked with Pbit 7
+## Configuration of pbit translatinon
+### 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 Pbit classification criteria, Device.QoS.Classification.{i}.EthernetPriorityCheck
+- Set Pbit rewrite 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.EthernetPriorityCheck 3
+obuspa -c set Device.QoS.Classification.1.EthernetPriorityCheck 7
+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_check '3'
+	option pcp_mark '7'
+```
+We can modify any of the params in the UCI file based on our preferance and use case and reload
+qos for it to take effect.