Update the v3 docs homepage

[djsauble]

The landing page of the 3.0 docs needs to be aligned with the new IA.

Closes https://github.com/PrefectHQ/prefect/issues/13994

Example

TODO

• [x] The Prefect Cloud, Upgrade to Prefect 3, Develop, Deploy, and React cards don't link to anything yet (they map to section titles, but you can't link to a section title)
• [x] The Sign up and API reference buttons have a blue bottom border which is inherited from a parent with the .prose class, and I'm not sure how to remove it without resorting to custom CSS. Any ideas @discdiver?
• [x] I feel like it would be cleaner to have a single class for turning a link into a primary (or secondary) button instead of having to assign ~10 individual tailwind classes to each <a> tag.
• [x] The code block is a bit large, and I think we should either simplify it, or else position it to the right of the buttons so that it doesn't take up so much vertical screen space.

[discdiver]

Thank you!

• While the section headings can't be linked to directly, the subsection headings are linkable and open the first element when navigated to. I think just linking to the first URL in the section could probably work.
• Custom CSS is what comes to mind for me, too. Will put you in touch with Mintlify folks. They might have another recomendaiton.
• Agree the code block could use a bit less padding.

I think that given the SDK location might be changing and it moves folks to another site, I'd remove the API reference button at the top of the page. Maybe the same with the Sign up button. I like the CTA idea. We have the CTA in the header and could put it lower in this page.