Fix docs social image URLs to be absolute ones

Prerequisites

• I have searched for duplicate or closed issues
• I have validated any HTML to avoid common problems
• I have read the contributing guidelines

Describe the issue

Broken after #36804

Reduced test cases

<meta name="twitter:image" content="/docs/5.3/assets/brand/bootstrap-social.png">

We need to conditionally construct the image URL and then pass it through Hugo's absURL function like before

What operating system(s) are you seeing the problem on?

Windows

What browser(s) are you seeing the problem on?

No response

What version of Bootstrap are you using?

unrelated