Docs: fix vertical alignment of header's LinkItems with icons

[julien-deramond]

Description

There's an issue with the vertical alignment of the header's LinkItems:

With the fix contained in this PR:

Adding align-items-center could have also be added directly in the LinkItem.astro file. But I preferred to add specifically this class at the right places.

Live previews

• https://deploy-preview-42041--twbs-bootstrap.netlify.app/

[mdo]

This one might need another pass—I'm working on navbar cleanup right now and the new .nav-link classes and more already require some other changes. My intention here is to require .nav on .navbar-nav, which would correctly set the alignment and padding and more for these nav links. Lemme get my navbar branch up and see if that covers all this.