Adds link a Gitlab.com account section

What does this MR do and why?

Adds a link your GitLab.com account section to the Customers Portal portion of the docs. Additionally, that gets referenced in the change linked namespace section to provide customers better guidance (specifically for sales-assisted purchases).

Closes https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5489

Screenshots or screen recordings

NA

How to set up and validate locally

NA

MR acceptance checklist

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