Add intersphinx extension for linking to other orgs (Qiskit/qiskit-metapackage#1769)

Adding intersphinx extension to allow for linking to other orgs in docs


I added links for `matplotlib.figure.Figure` return types to the
visualization docs in Qiskit#10389
, and @Eric-Arellano requested in
Qiskit#10389 (review)
to make that change here as well.

It didn't appear that the intersphinx extension was being used in this
repo (I didn't find it in `docs/conf.py`), so I added the extension and
all the mappings from qiskit-terra. Let me know if that's not the
correct approach to take here.

---------

Signed-off-by: Paul S. Schweigert <[email protected]>

Author: psschwei

docs/conf.py

@@ -58,6 +58,7 @@
     "matplotlib.sphinxext.plot_directive",
     "qiskit_sphinx_theme",
     "sphinx.ext.doctest",
+    "sphinx.ext.intersphinx",
 ]
 
 nbsphinx_timeout = 300
@@ -136,6 +137,14 @@
     ),
 }
 
+intersphinx_mapping = {
+    "rustworkx": ("https://qiskit.org/ecosystem/rustworkx/", None),
+    "qiskit-ibm-runtime": ("https://qiskit.org/ecosystem/ibm-runtime/", None),
+    "qiskit-aer": ("https://qiskit.org/ecosystem/aer/", None),
+    "numpy": ("https://numpy.org/doc/stable/", None),
+    "matplotlib": ("https://matplotlib.org/stable/", None),
+}
+
 # -- Options for HTML output -------------------------------------------------
 
 html_theme = "qiskit_sphinx_theme"