Add #![deny(clippy::undocumented_unsafe_blocks)] lint rule to enforce all unsafe are documented

[jerrysxie]

This generates tons of warnings which has to be dealt with.

[jerrysxie]

@JamesHuard @felipebalbi @kurtjd @madeleyneVaca

I was able to fix up most of the lint warnings. However, there is one last one I can't easily fix in embassy_hal_internal::interrupt_mod!

warning: docs for unsafe trait missing `# Safety` section

pub trait Interrupt: SealedInterrupt {
            /// Interrupt enum variant.
            ///
            /// This allows going from typelevel interrupts (one type per interrupt) to
            /// non-typelevel interrupts (a single `Interrupt` enum type, with one variant per interrupt).
            const IRQ: super::Interrupt;
            /// Enable the interrupt.
            #[inline]
            unsafe fn enable() {                                   <----------------------------
                Self::IRQ.enable()
            }

[kurtjd]

Yeah I've noticed running cargo clippy locally always generated this warning (even before introducing #![deny(clippy::undocumented_unsafe_blocks)]) but the CI pipeline filters it out somehow.

The best long-term approach is likely to see about getting an upstream patch submitted to Embassy adding # Safety comments to whatever might be missing it in the code generated by the interrupt_mod! macro, however a hacky workaround solution that seems to work is:

pub use interrupts::*;
/// Wrapper module to suppress clippy warning caused by macro.
#[allow(clippy::missing_safety_doc)]
pub mod interrupts {
    embassy_hal_internal::interrupt_mod!(
        ...
    );
}

Unfortunately can't just directly apply the #[allow] directive directly on top of the macro, have to first wrap it in a block (such as a module) before doing that (which is why we then export everything in the dummy interrupts module to not break existing code).