docs: update marketing pages guide examples (#479)

Rename the Proxy function to `proxy` and simplify the `identify` example
with the `Identify` type helper.

Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>
Co-authored-by: Dominik Ferber <1765075+dferber90@users.noreply.github.com>