Add warning to Namespace page about using '.' in the name

What does this MR do and why?

This MR updates the Namespace documentation with a challenge which one of our customer raised. This customer advised when using a . in their Namespace on GitLab.com they experience challenges getting the SSL certificate to validate the URL. Additionally they experience challenges with Terraform modules not recognising the source address.

Customer Ticket: https://gitlab.zendesk.com/agent/tickets/359263

Screenshots or screen recordings

N/A

How to set up and validate locally

Setup SSL with a Namespace that has a . in it. Setup Terrform modules to the Namespace.

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.