docs: add warning for GitHub SSH key deletion

assets/chezmoi.io/docs/reference/templates/github-functions/gitHubKeys.md

@@ -13,6 +13,14 @@ The returned value is a slice of structs with `.ID` and `.Key` fields.
     by using the `-v` / `--verbose` option when running `chezmoi apply` or
     `chezmoi update`.
 
+    Additionally, GitHub automatically [removes keys which haven't been used in
+    the last
+    year](https://docs.github.com/en/authentication/troubleshooting-ssh/deleted-or-missing-ssh-keys).
+    This may cause your keys to be removed from `~/.ssh/authorized_keys`
+    suddenly, and without any warning or indication of the removal. You should
+    provide one or more keys in plain text alongside this function to avoid
+    unknowingly losing remote access to your machine.
+
 !!! example
 
     ```