Add support for inline <kbd> elements

Hey all! First, let me just say that I absolutely love Outline and I'm so very appreciative of this product. It's absolutely wonderful and it's 99.99% perfect. However... I've got one small annoyance that I'm looking to hopefully resolve: When writing documentation, it would be very helpful to have support for <kbd> elements. I'd love to see a button in the formatting floater and a hotkey to wrap selected text in a <kbd> element. For those not in the know, [<kbd> is an HTML element]( that represents keyboard (or other text) input within inline text. As far as I'm aware, there is no Markdown character sequence to represent this element but many popular Markdown parsers will leave harmless inline HTML elements intact (including the parser we use at my organization for publishing our documentation). This would be an exceptionally useful formatting option for writing documentation, for tutorial authors, and other folks. An e…