Add building from source entry in docs (#1181)

## Description

Updates the docs by adding a brief section about building from source,
pointing out submodule initialization as an important step.

### Introduces a breaking change?

- [ ] Yes
- [x] No

### Type of change

- [ ] Bug fix (change which fixes an issue)
- [ ] New feature (change which adds functionality)
- [x] Documentation update (improves or adds clarity to existing
documentation)
- [ ] Other (chores, tests, code style improvements etc.)

### Tested on

- [ ] iOS
- [ ] Android

### Testing instructions

<!-- Provide step-by-step instructions on how to test your changes.
Include setup details if necessary. -->

### Screenshots

<!-- Add screenshots here, if applicable -->

### Related issues

<!-- Link related issues here using #issue-number -->

### Checklist

- [ ] I have performed a self-review of my code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [x] I have updated the documentation accordingly
- [ ] My changes generate no new warnings

### Additional notes

<!-- Include any additional information, assumptions, or context that
reviewers might need to understand this PR. -->

Author: IgorSwat

docs/docs/01-fundamentals/01-getting-started.md

@@ -184,6 +184,19 @@ Running the app with the library:
   </TabItem>
 </Tabs>
 
+## Building from source
+
+To build the library from source instead, clone the repository and initialize submodules:
+
+```bash
+git clone -b release/0.9 https://github.com/software-mansion/react-native-executorch.git
+cd react-native-executorch
+
+git submodule update --init --recursive packages/react-native-executorch/third-party/common
+
+yarn
+```
+
 ## Supporting new models in React Native ExecuTorch
 
 Adding new functionality to the library follows a consistent three-step integration pipeline: