Chore/rm main entity concept (#7709)

**What**
Update the `MedusaService` class, factory and types to remove the concept of main modules. The idea being that all method will be explicitly named and suffixes to represent the object you are trying to manipulate.
This pr also includes various fixes in different modules

Co-authored-by: Stevche Radevski <4820812+sradevski@users.noreply.github.com>
Co-authored-by: Oli Juhl <59018053+olivermrbl@users.noreply.github.com>

www/apps/resources/app/commerce-modules/fulfillment/concepts/page.mdx

@@ -13,7 +13,7 @@ A fulfillment set is a general form or way of fulfillment. For example, shipping
 A fulfillment set is represented by the [FulfillmentSet data model](/references/fulfillment/models/FulfillmentSet). All other configurations, options, and management features are related to a fulfillment set, in one way or another.
 
 ```ts
-const fulfillmentSets = await fulfillmentModuleService.create(
+const fulfillmentSets = await fulfillmentModuleService.createFulfillmentSets(
   [
     {
       name: "Shipping",

www/apps/resources/app/commerce-modules/fulfillment/page.mdx

@@ -88,7 +88,7 @@ const shippingOption =
 Support various fulfillment forms, such as shipping or pick up.
 
 ```ts
-const fulfillmentSets = await fulfillmentModuleService.create(
+const fulfillmentSets = await fulfillmentModuleService.createFulfillmentSets(
   [
     {
       name: "Shipping",