Update the documentation link for cothread

This is required due to a side-effect of making the /stable endpoint
also redirect to the GHPages version of the docs - the objects.inv link
does not correctly survive the redirect.

docs/conf.py

@@ -86,7 +86,9 @@
 # docs in the python documentation.
 intersphinx_mapping = dict(
     python=('https://docs.python.org/3/', None),
-    cothread=("https://cothread.readthedocs.org/en/stable/", None),
+    cothread=(
+        "https://diamondlightsource.github.io/cothread/master/", None
+    ),
     aioca=("https://dls-controls.github.io/aioca/master/", None),
     epicsdbbuilder=(
         "https://dls-controls.github.io/epicsdbbuilder/master/", None)