Add GH authentication reference page

Author: mfisher87

reference/03-gh-auth.md

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+# 🔐 Configuring GitHub authentication
+
+GitHub authentication on shared cloud infrastructure is complicated.
+
+Always remember, "cloud" is the same as "someone else's computer".
+It's not a good idea to store your credentials on someone else's computer!
+
+For this reason, we use a special tool called `gh-scoped-creds` to provide a special
+credential which grants restricted access to only certain GitHub resources for a limited
+amount of time.
+
+
+## Copy the GitHub authentication Notebook
+
+There's a special notebook titled `GHAUTH.ipynb` in the `shared/` directory.
+
+Right-click it, select "**Copy**", then return to your home directory by clicking the 📁
+icon at the top of your file browser.
+Finally, right click in the empty space in your home directory and select "**Paste**".
+
+:::{hint}
+Right-click this notebook and select "Add Favorite" to enable quick access in the
+future!
+:::
+
+
+## Follow the instructions in the Notebook
+
+**The first time you use this Notebook, you should open the "First time setup" hint and
+follow the instructions within.**
+
+After you've completed first-time setup, run the code cell at the end of the Notebook
+and carefully follow the instructions output by this cell.