docs: added troubleshooting guides + improvements (#11927)

* docs: added troubleshooting guides + improvements

* build fixes

www/apps/book/app/learn/fundamentals/modules/db-operations/page.mdx

@@ -28,8 +28,8 @@ So, to run database queries in a service:
 For example, in your service, add the following methods:
 
 export const methodsHighlight = [
-  ["12", "getCount", "Retrieves the number of records in `my_custom` using the `count` method."],
-  ["19", "getCountSql", "Retrieves the number of records in `my_custom` using the `execute` method."]
+  ["13", "getCount", "Retrieves the number of records in `my_custom` using the `count` method."],
+  ["20", "getCountSql", "Retrieves the number of records in `my_custom` using the `execute` method."]
 ]
 
 ```ts highlights={methodsHighlight}
@@ -38,7 +38,8 @@ import {
   InjectManager,
   MedusaContext,
 } from "@medusajs/framework/utils"
-import { SqlEntityManager } from "@mikro-orm/knex"
+import { Context } from "@medusajs/framework/types"
+import { EntityManager } from "@mikro-orm/knex"
 
 class BlogModuleService {
   // ...
@@ -46,19 +47,19 @@ class BlogModuleService {
   @InjectManager()
   async getCount(
     @MedusaContext() sharedContext?: Context<EntityManager>
-  ): Promise<number> {
-    return await sharedContext.manager.count("my_custom")
+  ): Promise<number | undefined> {
+    return await sharedContext?.manager?.count("my_custom")
   }
   
   @InjectManager()
   async getCountSql(
     @MedusaContext() sharedContext?: Context<EntityManager>
   ): Promise<number> {
-    const data = await sharedContext.manager.execute(
+    const data = await sharedContext?.manager?.execute(
       "SELECT COUNT(*) as num FROM my_custom"
     ) 
     
-    return parseInt(data[0].num)
+    return parseInt(data?.[0].num || 0)
   }
 }
 ```
@@ -115,8 +116,8 @@ class BlogModuleService {
     },
     @MedusaContext() sharedContext?: Context<EntityManager>
   ): Promise<any> {
-    const transactionManager = sharedContext.transactionManager
-    await transactionManager.nativeUpdate(
+    const transactionManager = sharedContext?.transactionManager
+    await transactionManager?.nativeUpdate(
       "my_custom",
       {
         id: input.id,
@@ -127,7 +128,7 @@ class BlogModuleService {
     )
 
     // retrieve again
-    const updatedRecord = await transactionManager.execute(
+    const updatedRecord = await transactionManager?.execute(
       `SELECT * FROM my_custom WHERE id = '${input.id}'`
     )
 
@@ -178,10 +179,22 @@ For example, the `update` method could be changed to the following:
 
 ```ts
 // other imports...
+import { 
+  InjectManager,
+  InjectTransactionManager,
+  MedusaContext,
+} from "@medusajs/framework/utils"
+import { Context } from "@medusajs/framework/types"
 import { EntityManager } from "@mikro-orm/knex"
 
 class BlogModuleService {
   // ...
+  @InjectTransactionManager()
+  protected async update_(
+    // ...
+  ): Promise<any> {
+    // ...
+  }
   @InjectManager()
   async update(
     input: {
@@ -192,12 +205,14 @@ class BlogModuleService {
   ) {
     const newData = await this.update_(input, sharedContext)
 
-    await sendNewDataToSystem(newData)
+    // example method that sends data to another system
+    await this.sendNewDataToSystem(newData)
 
     return newData
   }
 }
 ```
+
 In this case, only the `update_` method is wrapped in a transaction. The returned value `newData` holds the committed result, which can be used for other operations, such as passed to a `sendNewDataToSystem` method.
 
 ### Using Methods in Transactional Methods
@@ -208,6 +223,11 @@ For example:
 
 ```ts
 // other imports...
+import { 
+  InjectTransactionManager,
+  MedusaContext,
+} from "@medusajs/framework/utils"
+import { Context } from "@medusajs/framework/types"
 import { EntityManager } from "@mikro-orm/knex"
 
 class BlogModuleService {
@@ -343,7 +363,7 @@ class BlogModuleService {
         return updatedRecord
       },
       {
-        transaction: sharedContext.transactionManager,
+        transaction: sharedContext?.transactionManager,
       }
     )
   }
@@ -382,6 +402,12 @@ The second parameter of the `baseRepository_.transaction` method is an object of
 ```ts highlights={[["16"]]}
 // other imports...
 import { EntityManager } from "@mikro-orm/knex"
+import { 
+  InjectTransactionManager,
+  MedusaContext,
+} from "@medusajs/framework/utils"
+import { Context } from "@medusajs/framework/types"
+import { EntityManager } from "@mikro-orm/knex"
 
 class BlogModuleService {
   // ...
@@ -398,7 +424,7 @@ class BlogModuleService {
         // ...
       },
       {
-        transaction: sharedContext.transactionManager,
+        transaction: sharedContext?.transactionManager,
       }
     )
   }
@@ -414,6 +440,12 @@ class BlogModuleService {
 
 ```ts highlights={[["19"]]}
 // other imports...
+import { 
+  InjectTransactionManager,
+  MedusaContext,
+} from "@medusajs/framework/utils"
+import { Context } from "@medusajs/framework/types"
+import { EntityManager } from "@mikro-orm/knex"
 import { IsolationLevel } from "@mikro-orm/core"
 
 class BlogModuleService {
@@ -442,6 +474,14 @@ class BlogModuleService {
     - If `transaction` is provided and this is disabled, the manager in `transaction` is re-used.
 
 ```ts highlights={[["16"]]}
+// other imports...
+import { 
+  InjectTransactionManager,
+  MedusaContext,
+} from "@medusajs/framework/utils"
+import { Context } from "@medusajs/framework/types"
+import { EntityManager } from "@mikro-orm/knex"
+
 class BlogModuleService {
   // ...
   @InjectTransactionManager()