DOC: Use scientific-python-nightly-wheels for nightly build index

* The Scientific Python org is requesting that users of the
  https://pypi.anaconda.org/scipy-wheels-nightly/ package index
  migrate nightly builds to the new package index
  https://pypi.anaconda.org/scientific-python-nightly-wheels/
  which will have community support moving forward.
   - c.f. https://scientific-python.org/specs/spec-0004/
   - c.f. https://github.com/matplotlib/matplotlib/ PR 25950

Author: matthewfeickert

doc/users/installing/index.rst

@@ -85,17 +85,17 @@ Installing a nightly build
 ==========================
 
 Matplotlib makes nightly development build wheels available on the
-`scipy-wheels-nightly Anaconda Cloud organization
-<https://anaconda.org/scipy-wheels-nightly>`_.
-These wheels can be installed with ``pip`` by specifying scipy-wheels-nightly
-as the package index to query:
+`scientific-python-nightly-wheels Anaconda Cloud organization
+<https://anaconda.org/scientific-python-nightly-wheels>`_.
+These wheels can be installed with ``pip`` by specifying
+scientific-python-nightly-wheels as the package index to query:
 
 .. code-block:: sh
 
   python -m pip install \
     --upgrade \
     --pre \
-    --index-url https://pypi.anaconda.org/scipy-wheels-nightly/simple \
+    --index-url https://pypi.anaconda.org/scientific-python-nightly-wheels/simple \
     --extra-index-url https://pypi.org/simple \
     matplotlib