docs: document module providers in plugins (#11360)

* docs: document module providers in plugins

* small fixes

* add missing title

www/apps/resources/references/file/classes/file.AbstractFileProviderService/page.mdx

@@ -10,9 +10,18 @@ In this document, you’ll learn how to create a file provider module and the me
 
 ---
 
-## 1. Create Module Directory
+## Create Module Provider Directory
 
-Start by creating a new directory for your module. For example, `src/modules/my-file`.
+Start by creating a new directory for your module provider.
+
+If you're creating the module provider in a Medusa application, create it under the `src/modules` directory. For example, `src/modules/my-file`.
+If you're creating the module provider in a plugin, create it under the `src/providers` directory. For example, `src/providers/my-file`.
+
+<Note>
+
+The rest of this guide always uses the `src/modules/my-file` directory as an example.
+
+</Note>
 
 ---
 
@@ -251,6 +260,7 @@ module.exports = defineConfig({
             id: "local",
           },
           {
+            // if module provider is in a plugin, use `plugin-name/providers/my-file`
             resolve: "./src/modules/my-file",
             id: "my-file",
             options: {