Add missing root border opacity CSS var

Description

It looks like there is an issue with the latest modifications of the border colors on the main branch. Let's add .border-warning in the documentation when .border is not used:

### Additive

{{< example class="bd-example-border-utils" >}}
<span class="border border-warning"></span>
<span class="border-top border-warning"></span>
<span class="border-end border-warning"></span>
<span class="border-bottom border-warning"></span>
<span class="border-start border-warning"></span>
{{< /example >}}

### Subtractive

{{< example class="bd-example-border-utils bd-example-border-utils-0" >}}
<span class="border-0 border-warning"></span>
<span class="border-top-0 border-warning"></span>
<span class="border-end-0 border-warning"></span>
<span class="border-bottom-0 border-warning"></span>
<span class="border-start-0 border-warning"></span>
{{< /example >}}

Here is the rendering:

We can see in the browser that --bs-border-opacity is not set in those cases:

This PR is a proposal to define a default value for this CSS var in _root.scss.

Note: if this modification is correct, I didn't know if you prefer to have the value stored in $border-opacity or not.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

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

Related issues

N/A

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