Fix notebook links for stable version (icaros-usc#164)

docs/_templates/sourcelink.html

@@ -13,9 +13,13 @@
   <a class="md-nav__extra_link"
      {# Insert a Colab link for all Python notebooks. Notebooks are assumed to
      be under the docs folder -- unless they are tutorials, in which case they
-     are under the examples folder. #}
+     are under the examples folder. If "stable" is in the docstitle, then this
+     is a stable release, so the tutorial link should point to the version tag
+     on GitHub. Otherwise, this is the "latest" release, so we point to master.
+     #}
      href="{{
-       "https://colab.research.google.com/github/icaros-usc/pyribs/blob/master/" +
+       "https://colab.research.google.com/github/icaros-usc/pyribs/blob/" +
+       ("master/" if docstitle.find("stable") == -1 else "v" + version + "/") +
        ("examples/" if sourcename.startswith("tutorials/") else "docs/") +
        sourcename[:-4]
      }}">