Docs: what problem is NullBroadcasts solving

[Sbozzolo]

(I know documentation is under construction)

The current documentation describes the package, but does really mention where/why one would use `NullBroadcasted.

It'd be useful to add a line to provide more motivation for the package.

This package defines NullBroadcasted(), which can be added to, subtracted from, or multiplied by any value in a broadcast expression without incurring a runtime performance penalty. This can be useful for X

[charleskawczynski]

NullBroadcasts provides an elegant solution to summing/subtracting non-existing broadcasted objects in broadcast expressions, without a performance penalty. It will also allow us to skip kernel launches on simple increments, for example: @. x += NullBroadcasted(), which will result in Base.materialize!(dest, ::NullBroadcasted) = nothing (no cost, no reads/writes, no kernel launches, a no-op).

The original alternative we had in ClimaAtmos was to use something of the flavor @. x += zero(eltype(x)), but that means that we would be reading and writing to x for every point in space, in a useless operation. We don't want to pay for this. Enter NullBroadcasts.jl.

I guess we could show users how we plan to use it, if you think it's helpful. See, for example CliMA/ClimaAtmos.jl#3540.

[charleskawczynski]

Also, this helps us avoid issues with operations like * and + that using zero would face:

julia> 0 * NaN
NaN