Add documentation for work item status Iteration 2 GA

Add a separate documentation page for status for work items for example doc/user/work_items/status.md.

TBD whether we call this "customizable status" or just "status" and make sure we hide the technical implementation details at the higher level and maybe touch base on some specifics (about system-defined statuses and custom statuses) in one section.

Update existing documentation:

1. /status quick action: https://docs.gitlab.com/user/project/quick_actions/#work-items
2. Legacy issue boards: https://docs.gitlab.com/user/project/issue_board/
3. Available filters for work items: https://docs.gitlab.com/user/work_items/#available-filters
4. Fields in the new issue form: https://docs.gitlab.com/user/project/issues/create_issues/#fields-in-the-new-issue-form
5. Bulk edit issues: https://docs.gitlab.com/user/project/issues/managing_issues/#bulk-edit-issues-from-a-project
6. Manage issues: https://docs.gitlab.com/user/project/issues/managing_issues/ - maybe add a section about reopening and closing issues automatically based on a given status set