Docs: Fix clipboard code

[patrickhlauke]

Description

Fix clipboard code to work with fancy code blocks.

Motivation & Context

Corrects reported bug by @julien-deramond

There's an issue to fix before releasing alpha3 (Firefox macOS, IDK about other combinations) with the current main branch regarding "Copy to clipboard" button which doesn't work anymore (e.g. https://twbs-bootstrap.netlify.app/docs/5.3/components/accordion/#example or https://twbs-bootstrap.netlify.app/docs/5.3/components/accordion/#variables).
Due to 1729bcc
Error in the console:

 Uncaught TypeError: trigger.parentNode.nextElementSibling is null
   text http://localhost:9001/docs/5.3/assets/js/docs.js:95
   value http://localhost:9001/docs/5.3/assets/js/docs.js:7
   listener http://localhost:9001/docs/5.3/assets/js/docs.js:7
   i http://localhost:9001/docs/5.3/assets/js/docs.js:7

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-38380--twbs-bootstrap.netlify.app/docs/5.3/components/accordion/#variables