Snippets landing page

[rycoll]

It might be nice to have a page that talks about snippets generally. Currently, we just have a few articles about how to configure snippets for different SSGs, but I don't think there's anything that explains the idea behind snippets in much detail 🤔

At the very least, it would be good to have a general page to link to, when talking about snippets, instead of picking an SSG-specific guide that might not be relevant to whoever is clicking on the link.