Doc: Index the documentation and add a search box to navigate the website quickly

[loky-lp]

As many other svelte libraries (and even the official svelte websites) have a searchable documentation I think that the lacks of it really undermine the amazing work put into the documentation.

I'm looking forward to implement it myself using something like flexsearch.

With the creation of this issue, I'm asking for permission to proceed and for a feedback on the chosen library.

PS. Thank for the amazing work

[bryanmylee]

To be honest, I just need to set up Docsearch with Algolia, but I've been swamped at work 😮‍💨 Let me get to this as soon as possible.