docs: Add description of modules directory to project structure summaries

Author: aklinker1

docs/guide/essentials/project-structure.md

@@ -30,9 +30,10 @@ Here's a brief summary of each of these files and directories:
 - `.wxt/`: Generated by WXT, it contains TS config
 - `assets/`: Contains all CSS, images, and other assets that should be processed by WXT
 - `components/`: Auto-imported by default, contains UI components
-- `composables/`: Auto-imported by default, contains composable functions for Vue
+- `composables/`: Auto-imported by default, contains source code for your project's composable functions for Vue
 - `entrypoints/`: Contains all the entrypoints that get bundled into your extension
-- `hooks/`: Auto-imported by default, contains hooks for React and Solid
+- `hooks/`: Auto-imported by default, contains source code for your project's hooks for React and Solid
+- `modules/`: Contains [local WXT Modules](/guide/essentials/wxt-modules) for your project
 - `public/`: Contains any files you want to copy into the output folder as-is, without being processed by WXT
 - `utils/`: Auto-imported by default, contains generic utilities used throughout your project
 - `.env`: Contains [Environment Variables](/guide/essentials/config/environment-variables)