Reorganize monorepo to include /packages and /playgrounds

[endigo9740]

As discussed in #2817, we should move towards a new monorep structure that separates component packages from their playground apps. Playgrounds being the sandbox app for consuming and testing the components and features before they are distributed or shared in the docs.

Each playground would utilize the most common meta-framework for that framework:

• React Components + Next.js Playground
• Svelte Components + SvelteKit Playground
• Vue Comonents + Nuxt Playground
• Solid.js Components + SolidStart Playground
• Core package (Tailwind/CSS styles) + Astro Playground (see Feature: Tailwind V4 #3189)

We'll update the monorepo structure to adhere to the following:

/sites
/packages
/playgrounds

Alternatively we can use the term /sandboxes or /repls if preferred. I have no strong feelings on this!