doc: Update for PyPI fable-library is the default (#216)

Author: dbrattli

docs/docs/python/build-and-run.md

@@ -9,26 +9,60 @@ Fable targets Python 3.12 or higher.
 
 Python 3.10 and 3.11 are deprecated.
 
+## Installing fable-library
+
+Fable Python requires the `fable-library` package to be installed from PyPI. This package contains the core runtime library that Fable-generated Python code depends on.
+
+:::info
+The fable-library is partially written in Rust for correctness. Installing from PyPI ensures you get pre-built binaries that are compatible with your Python version and system architecture.
+:::
+
+### Version Pinning
+
+It is important to pin fable-library to a version compatible with your Fable compiler version. If you install an incompatible version, your generated code may not work correctly.
+
+**For stable releases (e.g., Fable 5.x):**
+
+```bash
+pip install "fable-library>=5.0.0,<6.0.0"
+```
+
+or with Poetry (in `pyproject.toml`):
+
+```toml
+fable-library = ">=5.0.0,<6.0.0"
+```
+
+or with uv:
+
+```bash
+uv add "fable-library>=5.0.0,<6.0.0"
+```
+
+**For alpha/beta releases:** Pin to the exact version matching your Fable compiler. Note that PyPI uses a different naming convention (e.g., `5.0.0-alpha.17` becomes `5.0.0a17`):
+
+```bash
+pip install "fable-library==5.0.0a17"
+```
+
 ## Running Python Code
 
-When targeting python, you can use the output of Fable directly by running it with the python interpreter.
+When targeting Python, you can use the output of Fable directly by running it with the Python interpreter.
 
 For example:
 
 ```bash
 python3 Program.py
 ```
 
-## Publishing to PyPI
+## Custom fable-library Path
 
-If you want to publish a library to PyPI that uses Fable Python, you need to use `--fableLib` option to reference the pre-build Fable library:
-
-:::info
-This is because the Fable library is partially written in Rust and needs to be built for all architectures
-:::
+If you need to use a custom version of fable-library (e.g., for development or testing), you can use the `--fableLib` option:
 
 ```bash
-dotnet fable --lang python --fableLib fable-library
+dotnet fable --lang python --fableLib /path/to/custom/fable-library
 ```
 
-This will make any reference to the Fable library point to a package in the Python search path (e.g., site-packages) instead of the normally bundled library. Your package will then need to declare `fable-library` as a dependency so users can install it from PyPI.
+## Publishing to PyPI
+
+When publishing a library to PyPI that uses Fable Python, your package should declare `fable-library` as a dependency with appropriate version constraints so users get a compatible version automatically.