Add ref for betweenness centrality

Author: rossbar

doc/index.md

@@ -33,7 +33,7 @@ originating in the same repository. See {ref}`sec:degree` for details.
 
 ### Most *central* issues
 
-A listing of issues sorted by *betweenness centrality*.
+A listing of issues sorted by *betweenness centrality*[^centrality].
 See {ref}`sec:centrality` for details.
 
 ````{admonition} Click to show/hide table
@@ -43,6 +43,8 @@ See {ref}`sec:centrality` for details.
 ```
 ````
 
+[^centrality]: https://en.wikipedia.org/wiki/Betweenness_centrality
+
 ## Details
 
 ```{toctree}