Docs: add shift-color() usage example in sass customization page

[julien-deramond]

Description

This PR suggests adding a new .custom-element-3 example in our Customize > Sass > Colors page to show how to use shift-color().

When .custom-element-3 is applied to a <div>, here is the corresponding rendering:

Motivation & Context

This is an enhancement that could be useful to folks when they are using this Sass function to show that the weight must be set in percentage too.

Type of changes

• Docs enhancement (non-breaking change)

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
• (N/A) I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-39435--twbs-bootstrap.netlify.app/docs/5.3/customize/sass/#colors

Related issues

Closes #39432