BUG: Fix DIPY doc URL in Sphinx documentation config file

Fix DIPY doc URL in Sphinx documentation config file: the URL was
changed starting DIPY 1.8.0.

Fixes:
```
Warning, treated as error:
failed to reach any of the inventories with the following issues:
intersphinx inventory 'https://dipy.org/documentation/latest/objects.inv'
not fetchable due to
<class 'requests.exceptions.HTTPError'>: 404 Client Error:
Not Found for url: https://dipy.org/documentation/latest/objects.inv
make: *** [Makefile:65: html] Error 2
```

raised when running:
```
make -C docs/ SPHINXOPTS="-W" BUILDDIR="$HOME/docs" OUTDIR="${CURBRANCH:-html}" html
```

docs/conf.py

@@ -235,7 +235,7 @@
 # Example configuration for intersphinx: refer to the Python standard library.
 intersphinx_mapping = {
     "bids": ("https://bids-standard.github.io/pybids/", None),
-    "dipy": ("https://dipy.org/documentation/latest/", None),
+    "dipy": ("https://docs.dipy.org/stable", None),
     "matplotlib": ("https://matplotlib.org/stable", None),
     "nibabel": ("https://nipy.org/nibabel/", None),
     "nipype": ("https://nipype.readthedocs.io/en/latest/", None),