Docs: mention sliders in range docs

[monsonego]

Description

• mention that range inputs are also commonly called sliders
• add the term slider to the page description and overview text for better discoverability

Motivation & Context

Issue #38712 notes that people often search for this control using the term slider and may not realize it is documented under Range in Bootstrap.

This change keeps the page title and structure intact while adding the common term in two natural places so the docs are easier to find and understand.

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

• N/A (docs-only change; I did not generate a hosted preview link in this checkout)

Validation

px prettier --config site/.prettierrc.json --check site/src/content/docs/forms/range.mdx

• git diff --check

px cspell --no-progress site/src/content/docs/forms/range.mdx

• attempted a local docs build, but the project build wrapper did not produce actionable output in this Windows checkout

Related issues

Closes #38712