Add deletion warning to "add codespaces" page

Adding codespaces to a project seems to be the main landing place to
which we direct researchers when introducing them to codespaces.

Make the 30d retention policy very clear from the outset by copying the
pre-existing warning from the "how to use codespaces" page.

docs/getting-started/how-to/add-github-codespaces-to-your-project/index.md

@@ -1,5 +1,10 @@
 This page describes how to add GitHub Codespaces to your project.
 
+!!! warning
+    Codespaces are automatically deleted after a period of inactivity and any changes not pushed to the GitHub repo will be lost.
+    For the `opensafely` organization,
+    this period is 30 days.
+
 ## Does your project already have GitHub Codespaces?
 
 Newer projects already have GitHub Codespaces.