docs: add DuckDB capabilities errors and examples

[Xavees]

Description

Adds the missing Capabilities & labels, Error handling, and Usage examples
sections to the DuckDB provider documentation.

The documentation reflects the current DuckDB provider implementation and
keeps the existing provider-specific Packaging and Security sections.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Test addition or update

Related Issue

Closes #1100

Changes Made

• Added the DuckDB Capabilities & labels section based on getCapabilities()
  and getLabels().
• Documented provider error mapping and open-time errors.
• Added usage examples for SQL executed against a DuckDB file.
• Renumbered the following sections and updated affected section references.
• Kept the existing DuckDB-specific Packaging and Security documentation.

Testing

• I have tested this locally
• I have added/updated tests
• All existing tests pass

Ran:

git diff --check

The documentation check completed successfully.

Also ran:

bun run test

The test suite ran locally on Windows, but three unrelated existing test files
failed:

• tests/components/admin/OperationsTab.test.tsx
• tests/unit/launcher-utils.test.ts
• tests/unit/packaging-standalone-tarball.test.ts

This PR only changes docs/providers/duckdb.md.

Test Environment

• LibreDB Studio Version: current main
• Browser: N/A
• OS: Windows
• Node.js/Bun Version: Bun 1.3.14
• Database Type: DuckDB

Screenshots (if applicable)

N/A

Checklist

• My code follows the project's code style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• The required CI test job passes the 100% line-coverage gate (bun run test:coverage and bun run coverage:check)
• If I changed src/lib/db/providers/, I updated the matching docs/providers/ documentation and tests/integration/db/ tests in the same PR (provider triad)
• Any dependent changes have been merged and published

Additional Notes

This is a documentation-only change. No executable provider code was modified.
CI can verify the full test and coverage suite.

[cevheri]

thanks and welcome
closes #1100
merging shortly