fix clippy lints and made documentation more readable

[Luro02]

This PR does a lot of stuff:

• first I applied cargo fmt to make the code more readable (my editor automatically does this, when I save a file)
• I then fixed all clippy lints except for the following

warning: you have declared #[inline(always)] on to_usize. This is usually a bad idea (inline_always lint)

link

• I also went over the entire documentation of ArrayVec and made it more readable, by adding sections and removing redundant code in the examples