added common doc & print entity system plugin doc

Author: Hrithik-verma

docs/assets/images/component_fn.png

@@ -83,7 +83,8 @@ We will take help of components to find entity integer value like:
   <summary>b. search the entity by its name</summary>
   
    ```c++
-      std::string name = _ecm.Component<components::Name>(entity)->Data();
+      // find the entity (int value) which has Name Component with value as "xyz"
+      targetEntity = _ecm.EntityByComponents(components::Name("xyz"));
    ```
 
    ```c++
@@ -121,12 +122,26 @@ We will take help of components to find entity integer value like:
 
 ...``Cmd`` means to command to change that container value like ```LinearVelocityCmd```, ```WorldPoseCmd```, 	```VisualCmd```
 
+### Read
+<details>
+   <summary>a. Component</summary>
+
+  ```c++
+  //find Name component value of the entity(any entity_type eg model,link,world etc)
+  auto Name = _ecm.Component<components::Name>(this->targetEntity);
+  this->modelName = Name->Data();
+  ```
+
+</details>
+
+
 #### Write
  <details>
    <summary>a. SetComponentData</summary>
 
    ```c++
    const gz::math::Vector3d vel(0.0, 0.0, this->zVelocity);
+   //set the LinearVelocityCmd component value to be vel
    _ecm.SetComponentData<components::LinearVelocityCmd>(this->targetEntity,{vel});
    ```
  </details>
@@ -135,6 +150,7 @@ We will take help of components to find entity integer value like:
    <summary>b. mutable component</summary>
    
    ```c++
+   //set the LinearVelocityCmd component value to be vel
    auto velComp = _ecm.Component<components::LinearVelocityCmd>(this->targetEntity);
    velComp->Data() = vel;
    ```

docs/assets/images/entity_model_point.png

@@ -0,0 +1,117 @@
+## Common In All System Plugin
+
+
+
+1) Go to a standalone plugin folder:
+```bash
+cd <path-to>/gazebo-sim-plugins-tutorial/standalone_gz_sim_plugins/<plugin_name>
+```
+
+2) Build:
+```bash
+mkdir build && cd build
+cmake ..
+make
+cd ..
+```
+
+3) Export plugin path (so Gazebo Sim can find it):
+```bash
+export GZ_SIM_SYSTEM_PLUGIN_PATH=$(pwd)/build
+```
+
+4) Launch Gazebo Sim with an SDF:
+```bash
+gazebo sim -v 4 <sdf_file_path>.sdf
+```
+
+
+
+Notes:<br>
+- "-v 4" prints debug logs (useful while developing)<br>
+- If Gazebo can’t find the plugin, re-check GZ_SIM_SYSTEM_PLUGIN_PATH and that build succeeded.<br>
+
+<br>
+
+
+## Code related
+
+###  ```_entity``` of Configure()
+
+```_entity``` is the entity it attached to ```world,model,light``` etc 
+
+
+```c++
+void PrintEntitySystemPlugin::Configure(const Entity &_entity,
+    const std::shared_ptr<const sdf::Element> &_sdf,
+    EntityComponentManager &_ecm,
+    EventManager &/*_eventMgr*/)
+```
+
+
+![enity_world](assets/images/entity_world_point.png)
+
+```xml
+<world>
+
+    <plugin>  </<plugin>>
+
+</world>
+```
+so here ```_entity``` is world entity
+
+
+![model_entity](assets/images/entity_model_point.png)
+
+here it ```_entity``` is model entity
+
+
+### Plugin in XML
+
+need register the plugin with all the class it inhertited 
+```c++
+// Register the plugin with Gazebo Sim
+GZ_ADD_PLUGIN(gz::sim::systems::PrintEntitySystemPlugin,
+              gz::sim::System,
+              gz::sim::ISystemConfigure,
+              gz::sim::ISystemPreUpdate)
+```
+
+alias
+```c++
+GZ_ADD_PLUGIN_ALIAS(gz::sim::systems::PrintEntitySystemPlugin,
+                    "gz::sim::systems::PrintEntitySystemPlugin")
+```
+
+Important:
+only name=```gz::sim::systems::PrintEntitySystemPlugin``` allowed for plugin you can't put any other name because it how we define on the alis ```GZ_ADD_PLUGIN_ALIAS(...)```
+
+so 
+
+```
+<!-- custom plugin attach to world-->
+<plugin
+    filename="PrintEntitySystemPlugin"
+    name="gz::sim::systems::PrintEntitySystemPlugin">
+</plugin>
+
+```
+
+```filename= PrintEntitySystemPlugin``` 
+
+ or 
+
+```filename= libPrintEntitySystemPlugin.so``` 
+
+so no other name is allowed
+
+comes from CMakeList.txt
+
+```cmake
+add_library(PrintEntitySystemPlugin SHARED PrintEntitySystemPlugin.cc)
+set_property(TARGET PrintEntitySystemPlugin PROPERTY CXX_STANDARD 17)
+target_link_libraries(PrintEntitySystemPlugin
+  PUBLIC gz-plugin${GZ_PLUGIN_VER}::gz-plugin${GZ_PLUGIN_VER}
+  PUBLIC gz-sim${GZ_SIM_VER}::gz-sim${GZ_SIM_VER})
+```
+