Doc: fix offset content from fixed navbar when jumping to headings

Closes #36278 (closed)

This PR is a suggestion to fix the issue #36278 (closed).

I've reverted the change from https://github.com/twbs/bootstrap/commit/195440f2fb1e94c014a9cf08f3eae40f3d224620 to include what was there before.

Few changes:

• Kept the new comment
• Removed the media-breakpoint-up(md) rule
• Changed the value from 4rem to use the new value 5rem

Live previews examples

• https://deploy-preview-36280--twbs-bootstrap.netlify.app/docs/5.1/customize/overview/#overview
• https://deploy-preview-36280--twbs-bootstrap.netlify.app/docs/5.1/customize/overview/#csps-and-embedded-svgs
• https://deploy-preview-36280--twbs-bootstrap.netlify.app/docs/5.1/examples/#snippets
• https://deploy-preview-36280--twbs-bootstrap.netlify.app/docs/5.1/examples/#custom-components

/CC @mdo (maybe there are edge cases that I'm not aware of)