Add another error message and the fix in the pages troubleshooting section

What does this MR do and why?

Documenting what to do when getting an error message "is not a recognized provider" while deploying Gitlab pages.

Helped with two customer tickets:

• https://gitlab.zendesk.com/agent/tickets/253992
• https://gitlab.zendesk.com/agent/tickets/263504

How to set up and validate locally

1. Have a GitLab installation with pages enabled, where gitlab_rails['pages_object_store_enabled'] = true but no bucket details have been configured.
2. Attempt to deploy the page.
3. You would get is not a recognized provider
4. Comment it out or configure s3 bucket, then reconfigure.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.