docs(grid): document .col-auto in variable-width content

[RohithPariki]

Description

• Explicitly mentions .col-auto alongside col-{breakpoint}-auto in the Variable width content section of the Grid documentation (site/src/content/docs/layout/grid.mdx).
• Updates the variable-width code example to demonstrate .col-auto alongside .col-md-auto.
• Updates the Margin utilities section in the Columns documentation (site/src/content/docs/layout/columns.mdx) to clarify usage with variable-width columns like .col-auto.

Motivation & Context

In the grid documentation under "Variable width content", the text previously only mentioned col-{breakpoint}-auto classes and only demonstrated .col-md-auto. However, .col-auto was not explicitly introduced or searchable in the section, yet was used in subsequent examples such as the Columns margin utilities section.

Explicitly documenting .col-auto makes it clear that the breakpoint-less variant exists for sizing columns based on natural content width across all viewports.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-{your_pr_number}--twbs-bootstrap.netlify.app/docs/5.3/layout/grid/#variable-width-content
• https://deploy-preview-{your_pr_number}--twbs-bootstrap.netlify.app/docs/5.3/layout/columns/#margin-utilities

Related issues

Closes #36978