Documentation: Add SourceforgePackage to the build systems docs (#31473)

lib/spack/docs/build_systems.rst

@@ -62,11 +62,12 @@ on these ideas for each distinct build system that Spack supports:
 
    build_systems/bundlepackage
    build_systems/cudapackage
+   build_systems/custompackage
    build_systems/inteloneapipackage
    build_systems/intelpackage
-   build_systems/rocmpackage
-   build_systems/custompackage
    build_systems/multiplepackage
+   build_systems/rocmpackage
+   build_systems/sourceforgepackage
 
 For reference, the :py:mod:`Build System API docs <spack.build_systems>`
 provide a list of build systems and methods/attributes that can be

lib/spack/docs/build_systems/sourceforgepackage.rst

@@ -0,0 +1,55 @@
+.. Copyright 2013-2022 Lawrence Livermore National Security, LLC and other
+   Spack Project Developers. See the top-level COPYRIGHT file for details.
+
+   SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+.. _sourceforgepackage:
+
+------------------
+SourceforgePackage
+------------------
+
+``SourceforgePackage`` is a 
+`mixin-class <https://en.wikipedia.org/wiki/Mixin>`_. It automatically
+sets the URL based on a list of Sourceforge mirrors listed in
+`sourceforge_mirror_path`, which defaults to a half dozen known mirrors.
+Refer to the package source 
+(`<https://github.com/spack/spack/blob/develop/lib/spack/spack/build_systems/sourceforge.py>`__) for the current list of mirrors used by Spack.
+
+
+^^^^^^^
+Methods
+^^^^^^^
+
+This package provides a method for populating mirror URLs.
+
+**urls**
+
+    This method returns a list of possible URLs for package source.
+    It is decorated with `property` so its results are treated as
+    a package attribute.
+
+    Refer to 
+    `<https://spack.readthedocs.io/en/latest/packaging_guide.html#mirrors-of-the-main-url>`__
+    for information on how Spack uses the `urls` attribute during
+    fetching.
+
+^^^^^
+Usage
+^^^^^
+
+This helper package can be added to your package by adding it as a base
+class of your package and defining the relative location of an archive
+file for one version of your software.
+
+.. code-block:: python
+   :emphasize-lines: 1,3
+
+    class MyPackage(AutotoolsPackage, SourceforgePackage):
+        ...
+        sourceforge_mirror_path = "my-package/mypackage.1.0.0.tar.gz"
+        ...
+
+Over 40 packages are using ``SourceforcePackage`` this mix-in as of
+July 2022 so there are multiple packages to choose from if you want
+to see a real example.