Add self-profile instructions to compile time profiling guide

[JMS55]

Improve https://github.com/bevyengine/bevy/blob/main/docs/profiling.md#compile-time by adding instructions for e.g. CARGO_INCREMENTAL=0 RUSTC_WRAPPER= RUSTC_BOOTSTRAP=1 cargo rustc --package bevy_render -- -Z self-profile -Z self-profile-events=default,args.

Produces a file you can run through crox and then visualize with perfetto.