docs: clarify paid subscription handling during account conversion

[Sudaisib]

Description

This PR addresses issue #26047 by adding comprehensive information about
paid subscription handling during account conversion.

Changes Made

• Enhanced the "What happens when you convert your account" section
• Added explicit information about Docker Pro subscriptions
• Clarified that remaining subscription value does not carry over
• Included timing recommendation to align with billing cycles
• Added reference to Docker Support for subscription-specific inquiries

Issue

Closes #26047

Type of Change

• Documentation update
• Bug fix
• New feature

Testing

• Verified Markdown syntax
• Reviewed for clarity and completeness
• Ensured no broken links or references

Checklist

• My changes follow the style guidelines of this project
• I have performed a self-review of my own changes
• My changes address the issue completely

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