Docs: js-docs unindent automatically the code inside shortcode

[julien-deramond]

Description

Based on the magical Hugo stuff done by @louismaximepiton from f8a56da, this PR suggests using the same spell for the new js-docs shortcode in order to have the correct indentation.

Before: (e.g. https://twbs-bootstrap.netlify.app/docs/5.3/components/alerts/#live-example)

Now: (e.g. https://deploy-preview-38349--twbs-bootstrap.netlify.app//docs/5.3/components/alerts/#live-example)

Type of changes

• Enhancement (non-breaking change which adds 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
• (N/A) I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-38349--twbs-bootstrap.netlify.app//docs/5.3/components/alerts/#live-example
• https://deploy-preview-38349--twbs-bootstrap.netlify.app/docs/5.3/components/modal/#varying-modal-content
• https://deploy-preview-38349--twbs-bootstrap.netlify.app/docs/5.3/components/toasts/#live-example
• https://deploy-preview-38349--twbs-bootstrap.netlify.app/docs/5.3/getting-started/javascript/#sanitizer