docs: Improve context for Finalizer tasks section

[ehs208]

Moved the explanatory paragraph to the beginning of the section to provide better context and clarity about why Finalizer tasks are useful.

Fixes #34225

Context

The "Finalizer tasks" section previously started without context, making it harder for readers to understand their purpose and why they are important.
This PR moves the explanatory paragraph to the beginning of the section, improving readability and comprehension for users who are learning about Gradle task execution.

Contributor Checklist

• Review Contribution Guidelines.
• Make sure that all commits are signed off to indicate that you agree to the terms of Developer Certificate of Origin.
• Make sure all contributed code can be distributed under the terms of the Apache License 2.0, e.g. the code was written by yourself or the original code is licensed under a license compatible to Apache License 2.0.
• Check "Allow edit from maintainers" option in pull request so that additional changes can be pushed by Gradle team.
• Provide integration tests (under <subproject>/src/integTest) to verify changes from a user perspective. (Not applicable for docs change)
• Provide unit tests (under <subproject>/src/test) to verify logic. (Not applicable for docs change)
• Update User Guide, DSL Reference, and Javadoc for public-facing changes.
• Ensure that tests pass sanity check: ./gradlew sanityCheck. (Docs change, should pass)
• Ensure that tests pass locally: ./gradlew :docs:quickTest.

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• ❌ ❓ must be fixed
• 🤔 💅 should be fixed
• 💭 may be fixed
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[ov7a]

Thank you for your contribution!

This PR is complete and simple enough, it will be merged immediately.