Update Workspace Docs - validation rules section
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## Description
This is a followup to this [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/194504#note_2579080334). We want to format the validation rules section in the workspace docs as a table according to the documentation style guide: https://docs.gitlab.com/development/documentation/styleguide/#tables
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## Acceptance criteria
- [ ] `Validation rules` section formatted as a table in `doc/user/workspace/_index.md`
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