docs/conf.py: updates per RTD docs

RTD is rolling out some changes.  Per
https://about.readthedocs.com/blog/2024/07/addons-by-default/, these
are the changes we need to make.

Signed-off-by: Jeff Squyres <jeff@squyres.com>
(cherry picked from commit db7ff49)

docs/conf.py

@@ -129,6 +129,15 @@ def get_tarball_version(path, expr):
 if key in os.environ and os.environ[key] == 'True':
     print("OMPI: found ReadTheDocs build environment")
 
+    # Tell Jinja2 templates the build is running on Read the Docs
+    if "html_context" not in globals():
+        html_context = {}
+    html_context["READTHEDOCS"] = True
+
+    # Define the canonical URL if you are using a custom domain on
+    # Read the Docs
+    html_baseurl = os.environ.get("READTHEDOCS_CANONICAL_URL", "")
+
     rtd_v = os.environ['READTHEDOCS_VERSION']
     if os.environ['READTHEDOCS_VERSION_TYPE'] == 'external':
         # Make "release" be shorter than the full "ompi_ver" value.