Add product docs for Mobile Screens landing page

[markushi]

[getsantry]

Routing to @getsentry/product-owners-insights for triage ⏲️

[bcoe]

@markushi we talked about this in chat. But, I think now that we have the single "Mobile" landing page, we'd be better to focus on getting Screen Rendering polished, vs., the combined Mobile Screens page. Grouping everything together into Mobile Insights solved a similar problem I feel? (I could be convinced it's still worth combining App Starts, Screen Loads, and Rendering, but it feels like in the short-term getting Screen Rendering to GA is probably the highest impact we can have?).