feat: add documentation for changelog script

[Sonichigo]

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Description

• Please describe your changes: This pull request updates the documentation for Harness Database DevOps (DB DevOps) by adding a new guide on using custom scripts for database operations. The changes aim to improve the clarity and usability of the documentation.

  • New Guide on Custom Scripts: Added a comprehensive guide titled "Using Custom Script for Harness Database DevOps," covering the use of custom scripts for managing Liquibase changelogs, creating DB Schemas, and deploying schemas in pipelines. The guide includes examples, best practices, and step-by-step instructions. (docs/database-devops/use-database-devops/get-started/using-custom-script.md)

• Jira/GitHub Issue numbers (if any): https://harness.atlassian.net/browse/DBOPS-998

• Preview links/images (Internal contributors only): __________________

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[bot-gitexp-user]

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[bot-gitexp-user]

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[stephenatwell]

lgtm