docs: Add examples of using OnlineStats

Add one or more examples to the docs of how to use OnlineStats with DTables, such as in https://github.com/joshday/OnlineStatsBase.jl/pull/26#issue-975956277:

```julia
julia> fun = fit!
fit! (generic function with 4 methods)

julia> v = [reduce(fun, [1,2,3], init=Mean()) for _ in 1:3]
3-element Vector{Mean{Float64, EqualWeight}}:
Mean: n=3 | value=2.0
Mean: n=3 | value=2.0
Mean: n=3 | value=2.0

julia> reduce(fun, v, init=Mean())
Mean: n=9 | value=2.0
```

But where instead of `[1,2,3]`, we pass in a `DTable` and specify `cols` as the columns we want to reduce over.