[docs] fix small a11y issues

[geoffrich]

Keeping this separate from #8069 since these are smaller fixes instead of a larger design change.

• remove duplicate text in showcase/deploy links by setting alt="" on the images - e.g. the link was read out as "Vercel logo Vercel" or "Screenshot of pudding.cool pudding.cool", now it's simply "Vercel" and "pudding.cool"
• make banner a (resolves "all page content must be contained by landmarks warnings")
• move skip link in front of banner - it was almost entirely obscured
• treat focusing deployment links the same as hovering them (i.e. color in logo and change text color)
• label search input
• make CTRL+K a div instead of a label - it doesn't really work as a label to the search input. I considered linking it to the input with aria-describedby but that didn't feel correct.

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