Implement cconvert for SubArray

[nhz2]

This PR also documents cconvert as part of the strided array interface because it may be needed if the default cconvert cannot work.

Here is an example of the bug this change fixes, which is closely related to the bug in #59685

struct MyVec <: AbstractVector{UInt8} end
Base.size(::MyVec) = (8,)
Base.getindex(::MyVec, i::Int) = UInt8(i)
struct MyWrapper x::UInt64 end
Base.cconvert(::Type{Ptr{UInt8}}, ::MyVec) = Base.cconvert(Ref{MyWrapper}, MyWrapper(htol(0x0807060504030201)))
Base.unsafe_convert(::Type{Ptr{UInt8}}, x::Ref{MyWrapper}) = Ptr{UInt8}(Base.unsafe_convert(Ref{MyWrapper}, x))
Base.elsize(::Type{MyVec}) = 1
Base.strides(::MyVec) = (1,)
function has_4(v)
    r = @ccall memchr(v::Ptr{UInt8}, 0x04::Cint, length(v)::Csize_t)::Ptr{Nothing}
    r !== C_NULL
end
has_4(MyVec())
# true
has_4(view(MyVec(), 2:5))
# false