Resolve out of date information for Skeleton v2 docs

[endigo9740]

Link to the Page

https://www.skeleton.dev/

Describe the Issue (screenshots encouraged!)

Given the recent delay for Skeleton v3, I will devote some time to pass through and ensure the Skeleton v2 documentation is accurate and update to date. This may include but not be limited to the following:

• Update the banner on the home page to a more generic: "Preview Skeleton v3"
• Ensure all Onboarding onboarding information remains accurate and up to date
• List the specific technical requirements fo v2 - Tailwind v3 supported, v4 is not
• Link to optional migration instructions for Svelte 5
• Ensure all v3 links are correct and functional
• Replace custom font docs with Fontsource (mirror v3)
• Remove/update the Design Tokens video, which is currently broken
• Update Deprecations across all features

Feel free to add comments for additional requests below.