Provide more information of release cadence

What does this MR do?

Adds more information about the release cadences discussed here: gitlab-com/Product#50 (closed).

Related issues

Relates to: gitlab-com/Product#50 (closed).

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9221

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9221), CE-MR afterwards