Add info about anaconda test channel packages to conda section (#84)

Co-authored-by: Ralf Gommers <[email protected]>
Co-authored-by: Nathan Goldbaum <[email protected]>

docs/installing_cpython.md

@@ -122,11 +122,24 @@ and installing the free-threaded binaries is also possible:
     sudo apt-get install python3.13-nogil
     ```
 
-### Conda-forge
+### Conda Packages
 
-```bash
-mamba create -n nogil -c conda-forge python-freethreading
-```
+=== "Conda-forge"
+    ```bash
+    mamba create -n nogil -c conda-forge python-freethreading
+    ```
+
+=== "Anaconda Testing"
+    Anaconda's test channel includes the Python interpreter and ABI-compatible
+    builds of many common packages, like NumPy, Cython, Pandas, etc.  These
+    packages use the `python_abi` metapackage and should be compatible with
+    conda-forge:
+
+    ```bash
+    conda create -n nogil --override-channels -c ad-testing/label/py313 -c https://repo.anaconda.com/pkgs/main python-freethreading
+    ```
+
+    [Full list of Anaconda test packages built with free-threading ABI.](https://anaconda.org/ad-testing/repo?label=py313_nogil&type=any)
 
 ## Containers