docs: add header for compilation from source

docs/source/installation.mdx

@@ -21,6 +21,8 @@ To install from PyPI.
 pip install bitsandbytes
 ```
 
+## Alternative: Compiling from source
+
 To compile from source, you need CMake >= **3.22.1** and Python >= **3.8** installed. Make sure you have a compiler installed to compile C++ (gcc, make, headers, etc.). For example, to install a compiler and CMake on Ubuntu:
 
 ```bash