Add /sbin/nologin as a shell option (#3704)

## Description

This PR adds `/sbin/nologin` as a shell option for the compute cluster.
This is in preparation for implementing a 2-stage user expiry, where the
user account gets locked (prevented from logging in) before deletion.


## Checklist
- [x] I have read and understood the [WATcloud
Guidelines](https://cloud.watonomous.ca/docs/community-docs/watcloud/guidelines)
- [x] I have performed a self-review of my code

pages/docs/compute-cluster/machine-usage-guide.mdx

@@ -171,6 +171,16 @@ a dashboard to monitor the health of machines in the cluster.
 
 This section contains some common issues that users may encounter when using machines in the cluster and their solutions. If you encounter an issue that is not listed here, please [reach out](./support-resources).
 
+### Account not available
+
+You may see the following message when trying to log into a general-use machine:
+
+> This account is currently not available.
+
+This message means that your account is locked.
+This can happen if your account is expired and is pending deletion.
+To re-enable your account, please reach out to your [WATcloud contact](/docs/services#watcloud-contact).
+
 ### Permission denied while trying to connect to the Docker daemon
 
 You may encounter this error when trying to run Docker commands on general-use machines: