Updated Success Plans descriptions and process in the SA Handbook

Why is this change being made?

This merge request updates the content of several handbook pages related to customer success plans. It introduces a new "Mutual Customer Success Plan" concept, which is a customer-facing roadmap for achieving value through GitLab adoption. It also includes a detailed explanation of when and how to create a Mutual Customer Success Plan, as well as how it differs from a Technical Close Plan. Additionally, it provides a step-by-step guide on how to build a Technical Close Plan and transition it to a Customer Success Plan. This is part of the issue to rollout Success Plans across the org

CHANGEME

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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  • The when to get approval handbook section explains the workflow in more detail
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  • Team: For changes that affect your direct team, share in your group Slack channel
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Commits

• Update 4 files

• /content/handbook/solutions-architects/sa-practices/customer-success-plan/index.md
• /content/handbook/solutions-architects/sa-practices/technical-close-plan/_index.md
• /content/handbook/solutions-architects/sa-practices/_index.md
• /content/handbook/customer-success/csm/success-plans/_index.md

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