feat: mdx docs and tests (#61)

# Motivation
<!-- Why is this change necessary? -->

# Content
<!-- Please include a summary of the change -->
# Testing
<!-- How was the change tested? -->
# Please check the following before marking your PR as ready for review

- [ ] I have added tests for my changes
- [ ] I have updated the documentation or added new documentation as
needed
- [ ] I have read and agree to the [Contributor License
Agreement](../CLA.md)

---------

Co-authored-by: codegen-bot <[email protected]>

Author: jayhack

docs/building-with-codegen/files-and-directories.mdx

@@ -50,6 +50,32 @@ dir = file.directory
 exists = codebase.has_directory("path/to/dir")
 ```
 
+## Working with Non-Code Files (README, JSON, etc.)
+
+By default, Codegen focuses on source code files (Python, TypeScript, etc). However, you can access all files in your codebase, including documentation, configuration, and other non-code files like README.md, package.json, or .env:
+
+```python
+# Get all files in the codebase (including README, docs, config files)
+files = codebase.files(extensions="*")
+
+# Print files that are not source code (documentation, config, etc)
+for file in files:
+    if not file.filepath.endswith(('.py', '.ts', '.js')):
+        print(f"📄 Non-code file: {file.filepath}")
+```
+
+You can also filter for specific file types:
+
+```python
+# Get only markdown documentation files
+docs = codebase.files(extensions=[".md", ".mdx"])
+
+# Get configuration files
+config_files = codebase.files(extensions=[".json", ".yaml", ".toml"])
+```
+
+These APIs are similar for [`Directory`](../api-reference/core/Directory), which provides similar methods for accessing files and subdirectories.
+
 ## Raw Content and Metadata
 
 ```python

src/codegen/sdk/core/codebase.py

@@ -224,6 +224,8 @@ def files(self, *, extensions: list[str] | Literal["*"] | None = None) -> list[T
         if extensions is None:
             # Return all source files
             files = self.G.get_nodes(NodeType.FILE)
+        elif isinstance(extensions, str) and extensions != "*":
+            raise ValueError("extensions must be a list of extensions or '*'")
         else:
             files = []
             # Get all files with the specified extensions

tests/unit/codebase/file/test_file.py

@@ -59,6 +59,28 @@ def test_codebase_files(tmpdir) -> None:
         assert {f for f in codebase.files(extensions=[".bin"])} == {file3}
 
 
+@pytest.mark.skip("MDX editing is broken")
+def test_codebase_edit_mdx(tmpdir) -> None:
+    """Editing MDx seems broken currently - it will just prepend to the file"""
+    with get_codebase_session(tmpdir=tmpdir, files={"file1.mdx": "# Header", "file2.tsx": "console.log('hello, world!')"}) as codebase:
+        file = codebase.get_file("file1.mdx")
+        file.edit("NEW TEXT")
+        codebase.commit()
+        file = codebase.get_file("file1.mdx")
+        assert file.content == "NEW TEXT"
+
+
+@pytest.mark.skip("MDX replacing is broken")
+def test_codebase_replace_mdx(tmpdir) -> None:
+    """Editing MDx seems broken currently - it will just prepend to the file"""
+    with get_codebase_session(tmpdir=tmpdir, files={"file1.mdx": "# Header"}) as codebase:
+        file = codebase.get_file("file1.mdx")
+        file.replace("# Header", "NEW TEXT")
+        codebase.commit()
+        file = codebase.get_file("file1.mdx")
+        assert file.content == "NEW TEXT"
+
+
 @pytest.mark.skipif(sys.platform == "darwin", reason="macOS is case-insensitive")
 def test_file_extensions_ignore_case(tmpdir) -> None:
     with get_codebase_session(tmpdir=tmpdir, files={"file1.py": "print(123)", "file2.py": "print(456)", "file3.bin": b"\x89PNG", "file4": "Hello world!"}) as codebase: