docs(combo): add Blazing Story for IgbCombo with multiple scenarios

[dkamburov]

Description

This pull request adds a comprehensive set of Storybook stories for the IgbCombo component, covering a variety of usage scenarios and configurations. These stories demonstrate how to use the combo box with different props, selection modes, grouping, preselected values, disabled state, and numeric value keys. The stories also include example datasets and preselected values for demonstration purposes.

New Storybook stories for IgbCombo:

• Added stories for default usage, single select mode, grouped items, preselected values, disabled state, and numeric value keys, each showcasing different IgbCombo features and configurations.
• Defined example datasets (_cities, _citiesWithNumericIds) and preselected values (_selectedCities, _selectedCityIds) to support the stories.
• Introduced record types City and CityNumIds for structured data representation in the stories.

Motivation / Context

Type of Change (check all that apply):

• Bug fix
• New functionality
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (no functional changes)
• Documentation
• Demos
• CI/CD
• Tests
• Changelog

Component(s) / Area(s) Affected:

How Has This Been Tested?

• Unit tests
• Manual testing
• Automated e2e tests

Test Configuration:

• .NET version:
• Hosting model:
• Browser(s):
• OS:

Screenshots / Recordings

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code
• This PR includes API docs for newly added methods/properties
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes behavioral changes and the feature specification has been updated with them
• Accessibility (ARIA, keyboard navigation, focus management) has been verified

Closes #

[Copilot]

Pull request overview

Adds a new Blazing Storybook story file for IgbCombo, providing several usage scenarios to help consumers understand and validate common configurations of the combo component in the Ignite UI for Blazor story catalog.

Changes:

• Introduces Combo.stories.razor with multiple IgbCombo stories (default, single-select, grouped, preselected, disabled, numeric value keys).
• Adds in-file sample datasets and preselected values to support the stories.
• Defines simple record types to model the sample data used by the stories.

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