Fix 404s on outbound biojulia.dev links

[danielle-pinto]

Clicking links from https://biojulia.dev/dev/ to other sites hosted on the same domain (e.g. https://biojulia.dev/BioTutorials/dev/, https://biojulia.dev/TwoBit.jl/stable/) shows a 404 page until a refresh. This happens because the VitePress client router navigates to these links within the https://biojulia.dev/dev. However, these links don't have the /dev at the end of their URL (this is just added to the biojulia website during deployment)

This PR updates the external “feature card” links on the homepage (docs/src/index.md) to use target="_blank" and rel="noopener noreferrer", bypassing the VitePress router and ensuring the browser performs a real navigation. This makes sure that a new webpage is opened, rather than just navigating within https://biojulia.dev/dev/ . rel="noopener noreferrer" also prevents window.opener access for security. This prevents a user from clicking back. This isn't inherently related to the problem, but this is a common pattern that people use.

The old solution that was merged in (rel="external") didn't work. While this tells VitePress that the website is an external website, this is a piece of metadata. It doesn't tell Vitepress to change how it handles opening the link.

[kescobo]

I'm getting real "get a TUI to 60fps" vibes here...

https://youtu.be/LvW1HTSLPEk?si=dUJWFk3UB-hwkCvN

(This is not a criticism of you)

[kescobo]

Hey, it works!