Review and reduce global styles in the core

[endigo9740]

Describe the feature in detail (code, mocks, or screenshots encouraged)

Currently Skeleton introduces a number of @base styles by default when importing and using Skeleton's core package.

Goal

Find ways to remove or reduce the number of global styles. For any styles that remain, consider implementing a opt-in approach to enabling these. This may introduce a new onboarding step, but this is likely a necessary change.

Task: Update Documentation

Update the relevant Core API and Onboarding docs to account for this change.

Provide relevant links, wireframes, prototypes, or additional information.

• Docs: https://www.skeleton.dev/docs/svelte/get-started/core-api#base
• Source: https://github.com/skeletonlabs/skeleton/blob/main/packages/skeleton/src/base/globals.css