docs: add 'skeleton loaders' to loading placeholders description

[MohamadSalman11]

Description

Added 'skeleton loaders' as an alternative term for loading placeholders in docs

Motivation & Context

It’s also widely known as skeleton loaders

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-41177--twbs-bootstrap.netlify.app/docs/5.3/components/placeholders