Docs: set CLI as the new SvelteKit onboarding default

[endigo9740]

Link to the Page

https://www.skeleton.dev/guides/install

Describe the Issue

Let's update and tailor the onboarding documentation to recommend the CLI by default. We'll want to clearly indicate it's in beta, however, we've had zero issues reported since it went live. It seems to be doing everything we need it to do.

The challenge here will be balancing the onboarding user path. Essentially there's two options now:

1. SvelteKit users that use the CLI - they can skip straight to building their apps
2. SvelteKit/Vite/Astros that either bypass or can't use the CLI - they still need the written instruction

Additionally, some of the information in the written guide is beneficial to CLI users - such as how to set or create a custom theme, how to override styles and use component props, and just generally know what the CLI is doing for them.

Are you able to create a Pull Request with the fix?

Yes

[endigo9740]

NOTE: we'll want to implement a quick tab switcher for the CLI and support to deep link to the Vite/Astro tabs:

https://www.skeleton.dev/guides/install?framework=sveltekit|vite|astro