Documentation changes for SaaS rollout

What does this MR do and why?

Add all the documentation changes that will be need as a result of the subscription card new UI changes for SaaS (epic: [SaaS] New subscription card display (&10921 - closed)).

For details about documentation that needs to be updated, see customers-gitlab-com#6948 (closed).

Screenshots or screen recordings

Sections that has changed:

• Change linked namespace
• Add seats
• Upgrade your GitLab SaaS subscription tier
• Renew or change a GitLab SaaS subscription
• Enable or disable automatic subscription renewal

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.

Related to customers-gitlab-com#6948 (closed)