Documentation: Add database guides for NOT NULL constraints

What does this MR do?

This MR adds documentation and guidance related to NOT NULL constraints:

1. When to add NOT NULL constraints
2. How to add/remove NOT NULL constraints in migrations
3. How to update existing columns and run a data migration to clean up existing invalid records

It addresses the questions raised in #31833 (closed)

• should we be adding a constraint for presence validated fields?
• how do we approach the cleanup of problem data already on gitlab.com and in customer instances?

and covers all other use cases as well.

Related issues

Closes #31833 (closed)

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have developer access or higher (for example, GitLab team members or Core Team members)
  • Apply the documentation label, plus:
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  • Assign the designated Technical Writer.

When applicable:

• Update the permissions table.
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• Add/update the feature flag section.
• If you're changing document headings, search doc/*, app/views/*, and ee/app/views/* for old headings replacing with the new ones to avoid broken anchors.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
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  • Add docs-only when the only files changed are under doc/*.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.