docs(bundler): document runtime resource paths

[Hushlor]

Summary

• clarify that tauri-bundler packages resources but does not resolve their paths at runtime
• document tauri_utils::platform::resource_dir as the public helper for applications that use the bundler without the Tauri runtime
• explain platform-specific resource locations and distinguish generic OS directories from bundle-identifier-specific application directories
• align the Tauri runtime path documentation with the shared resource-directory helper

Testing

• cargo fmt --check -- crates/tauri-utils/src/platform.rs crates/tauri/src/path/desktop.rs
• cargo test -p tauri-utils --doc platform::resource_dir
• cargo test -p tauri-utils platform::tests::resolve_resource_dir
• compiled the exact README example as Rust metadata
• cargo doc -p tauri-utils --no-deps
• cargo doc -p tauri --no-deps
• git diff --check

The documentation builds emitted only pre-existing warnings outside the changed lines. Full-workspace tests and Clippy were not run because this change only updates documentation and examples. No .changes entry is included because the change does not require a version bump.

Fixes #15738