* docs: add installation instructions to README

Add inline installation steps for Sentry Wizard, Swift Package
Manager (Xcode UI and Package.swift), and pre-built XCFrameworks
so users can get started without leaving the repo.

* docs: clarify pre-built vs compile-from-source install

Point users to sentry-apple-binaries for pre-built static
binaries and clarify this repo is for compiling from source.