WIP: Add docs for GitLab.com subscription upgrades

Oswaldo Ferreirarequested to merge
docs-gitlab-com-subscription-upgrade into master

What does this MR do?

This briefly describes the process of how to upgrade a GitLab.com subscription.

It can't be merged before https://gitlab.com/gitlab-org/customers-gitlab-com/merge_requests/367!

Edited by Oswaldo Ferreira

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