docs: improvements to module and data model documentation (#8062)

* docs: improvements to module and data model documentation

* add note about data model name casing

www/apps/book/app/basics/data-models/page.mdx

@@ -39,7 +39,7 @@ export default MyCustom
 
 You define a data model using the `model`'s `define` method. It accepts two parameters:
 
-1. The first one is the name of the data model's table in the database.
+1. The first one is the name of the data model's table in the database. It should be in snake-case form.
 2. The second is an object, which is the data model's schema. The schema's properties are defined using the `model`'s methods.
 
 The example above defines the data model `MyCustom` with the properties `id` and `name`. 

www/apps/book/app/basics/modules-and-services/page.mdx

@@ -52,7 +52,9 @@ For example, create the file `src/modules/hello/index.ts` with the following con
 import HelloModuleService from "./service"
 import { Module } from "@medusajs/utils"
 
-export default Module("helloModuleService", {
+export const HELLO_MODULE = "helloModuleService"
+
+export default Module(HELLO_MODULE, {
   service: HelloModuleService,
 })
 ```
@@ -68,18 +70,21 @@ The last step is to add the module in Medusa’s configurations.
 
 In `medusa-config.js`, add a `modules` property and pass to it your custom module:
 
-```js title="medusa-config.js" highlights={[["4", "helloModuleService", "The key of the main service to be registered in the Medusa container."]]}
+```js title="medusa-config.js" highlights={[["7", "HELLO_MODULE", "The key of the main service to be registered in the Medusa container."]]}
+import { HELLO_MODULE } from "./src/modules/hello"
+// ...
+
 module.exports = defineConfig({
   // ...
   modules: {
-    helloModuleService: {
+    [HELLO_MODULE]: {
       resolve: "./modules/hello",
     },
   },
 })
 ```
 
-Its key (`helloModuleService`) is the name of the module’s main service. It will be registered in the Medusa container with that name. It should also be the same name passed as the first parameter to the `Module` function in the module's definition.
+Its key (`helloModuleService` or `HELLO_MODULE`) is the name of the module’s main service. It will be registered in the Medusa container with that name. It should also be the same name passed as the first parameter to the `Module` function in the module's definition.
 
 Its value is an object having the `resolve` property, whose value is either a path to module's directory relative to `src`(it shouldn't include `src` in the path), or an `npm` package’s name.
 
@@ -94,13 +99,14 @@ For example, create the API route `src/api/store/custom/route.ts` with the follo
 ```ts title="src/api/store/custom/route.ts"
 import { MedusaRequest, MedusaResponse } from "@medusajs/medusa"
 import HelloModuleService from "../../../modules/hello/service"
+import { HELLO_MODULE } from "../../../modules/hello"
 
 export async function GET(
   req: MedusaRequest,
   res: MedusaResponse
 ): Promise<void> {
   const helloModuleService: HelloModuleService = req.scope.resolve(
-    "helloModuleService"
+    HELLO_MODULE
   )
 
   res.json({