Docs: Add new docs for MR work item widget
## Problem to solve
Merge request [!221290](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/221290) adds a new "Work Items" widget to the MR sidebar that displays related work items linked via closing patterns (like `Closes #123`) and mentioned work items. This feature needs documentation updates to help users understand how to use it.
## Further details
The new widget displays work items in two categories:
- **Closing**: Work items linked via closing patterns in the MR description
- **Mentioned**: Work items referenced but not closing
The widget is collapsible when there are more than 2 items. Users can click on items to open a work item drawer for more details.
This feature helps users understand the relationship between merge requests and work items, making it easier to track which issues are being resolved by a particular MR.
## Proposal
Update documentation in the following locations:
1. **Primary**: [Work items](https://docs.gitlab.com/user/work_items/) documentation
- Add a section explaining how work items linked via closing patterns automatically appear in the MR sidebar widget
- Explain the "Closing" vs "Mentioned" categorization
- Link to the closing patterns documentation
2. **Secondary (optional)**: [Create merge requests](https://docs.gitlab.com/user/project/merge_requests/creating_merge_requests/)
- Add a tip about using closing patterns to link work items, which will then display in the MR sidebar
## Who can address the issue
Technical writer familiar with work items and merge request documentation.
## Other links/references
- MR: [!221290 - Add related issues widget to MR](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/221290)
- Related documentation: [Closing issues automatically](https://docs.gitlab.com/user/project/issues/managing_issues/#closing-issues-automatically)
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