fix: make IndexHtml public to bring back with_index_html()

[xenzh]

Rationale

• Dioxus 0.6.x allowed to inject a custom index.html string via ServeConfigBuilder::new().index_html(s)
• 0.7.x replaced it (resolve the index before rendering #4806, follow up to serveconfig #4820) with ServeConfig::with_index_html() that takes IndexHtml, but this type was left pub(crate).
• This change makes the type public along with three constructors (which are already pub).
• I've also replaced anyhow::Error with a dedicated type since now it's a part of public API.

I don't think this exposes anything unintended - but please let me know if I missed anything.

Testing

• All cargo checks from https://dioxuslabs.com/learn/0.7/beyond/contributing#before-you-contribute passed.
• I haven't run playwright tests since they seemed unrelated.
• Also confirmed this fixes my workflow (serving /public contents embedded into the server binary).