Docs: Add transparency option

[louismaximepiton]

Description

Adding an option in the navbar to toggle a specific background (disabled by default) to help people see the transparency or not of a component.

Motivation & Context

We were ready to add it to our documentation (which is based on the Bootstrap one) so I try it upstream before all to see if this kind of feature would be needed. Feel free to close the PR if not needed in here.

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-40779--twbs-bootstrap.netlify.app/docs/5.3/components/buttons#outline-buttons
• https://deploy-preview-40779--twbs-bootstrap.netlify.app/docs/5.3/components/list-group

Related issues

NA

[coliff]

This is a useful enhancement and implemented really well. On the current docs it's not always clear at a glance if an elements color is the default color or if it has partially transparency - with this change it's much clearer. 👍

Update: Agree that on some pages it has no effect and there's no feedback that anything has changed.. perhaps the button could be moved to the example block next to the Stackblitz button?

[XhmikosR]

I'm not sure we need a new button, I'm not in favor of this, because in most pages it's useless, it doesn't change anything and the button doesn't show any feedback that something has changed.

What we could probably have, though, is this effect more subtle and enabled by default.

But I'd like to hear @mdo's opinion before landing such a change :)