Document whether debug colors are HSLA or something else

[martin-t]

The doc comment on DebugColor says that it uses HSLA but DebugRenderBackend::draw_line takes [f32; 4] so it's not clear from the docs what format the color is meant to be. If rapier uses HSLA everywhere, it should probably use DebugColor consistently everywhere.

There's a couple more places in code where i saw [f32; 4] (such as sleep_color_multiplier).

[sebcrozet]

Yes, they are all HSLA by default. I agree that we should use DebugColor everywhere.

[MaxBittker]

these docs currently use them as rgba: https://rapier.rs/docs/user_guides/javascript/getting_started_js/