Draft: Collate work items development resources

What does this MR do and why?

We want to reorganize the documentation structure for work items by moving files into a dedicated work_items folder and updating all the related links throughout the codebase.

This is essentially a housekeeping change that better organizes the work items documentation into a logical folder structure, making it easier for developers to find and navigate related documentation. All the actual content remains the same - only the file locations and links have been updated.

The links which can be put together

• https://docs.gitlab.com/development/work_items/
• https://docs.gitlab.com/development/work_items_widgets/
• https://docs.gitlab.com/api/graphql/epic_work_items_api_migration_guide/

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