Add auto-renew information to Self-Managed docs page

What does this MR do and why?

Related to https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5190

With the release of https://gitlab.com/groups/gitlab-org/-/epics/8319 epic, we'll be able to auto-renew self-managed subscriptions. Updating Docs to include auto-renewal.

I kept it mostly consistent with SaaS auto-renew information.

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