docs: new Getting Started page

[NikitaRevenco]

This is a part of a series of pull requests to port the new docs from https://helix-editor.vercel.app into the mdbook

This PR:

• Ports the "Getting Started" section
• Adds styles for kbd
• Adds CSS for custom asides, like aside class="tip". I think this is needed because I'm currently seeing that each "note" is hard coded

my plan at the moment is to make a separate PR for each of the 7 "Getting Started" pages on https://helix-editor.vercel.app one after the other

[TornaxO7]

Just some smol suggestions.

Hm... one though which I'm getting is: Don't we already have the helix-tutor? In my opinion it sounds a bit troublesome in the future if we have to maintain two "big" tutorials/guides if you know what I mean but on the other more information don't hurt and I think some people prefer a tutorial in the internet. Just my two cents.

[NikitaRevenco]

Just some smol suggestions.

Hm... one though which I'm getting is: Don't we already have the helix-tutor? In my opinion it sounds a bit troublesome in the future if we have to maintain two "big" tutorials/guides if you know what I mean but on the other more information don't hurt and I think some people prefer a tutorial in the internet. Just my two cents.

Yeah, I think having two tutorials won't hurt. The helix-tutor is more suitable for the editor than the web, I feel