Subscriptions doc page edits: clarify it's also for ongoing management, reduce duplication in Support contact info

What does this MR do?

Renames to Subscriptions and offers minor edit to the intro, both to clarify that this is for new and existing subscriptions.

Simplifies Contact Support section, relying more on existing SSOT support pages/links.

Related issues

gitlab-com/support/support-team-meta#1323 (closed)
https://gitlab.com/gitlab-org/gitlab-ce/issues/56875
https://gitlab.com/gitlab-org/gitlab-ce/issues/56876

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the ~Documentation label.

Review checklist

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3. Maintainer

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2. Ensure a release milestone is set and that you merge the equivalent EE MR before the CE MR if both exist.
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