Site: Clean up "Getting Started"

### Is your feature request related to a problem? Please describe.

Context is [this discussion](https://github.com/apache/polaris/pull/3550#discussion_r2852417571)

In #3550, which is primarily moving files "around" to make those accessible on the web site, the "Getting Started" guide became a pretty empty page. This issue is to track this and eventually have a nicer experience.

### Describe the solution you'd like

_No response_

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start by reading the current "Getting Started" guide and the context in discussion #3550, especially the file moves that left the page empty. Define what a useful guide should contain, then update the site content and verify that the page provides a complete getting-started experience.

Written by the indexing model from the issue text.