Improve docs for some functions in the Series submodule

[kimjoaoun]

This pull request adds examples for the following functions Series.(first, add, last, multiply, subtract, distinct).

[cigrainger]

Thanks for this! Looks great. There is at least one failing test that looks like a spacing issue.

[kimjoaoun]

@cigrainger it is not a spacing issue, it is because Explorer.Series.distinct() do not maintain the order. Each execution returns a different result so the test always fails.

I removed the iex> calls from the distinct docs and now the tests should pass. I think the fast solution to this problem is enforcing a seed as we can see here, an optimal one would be making Series.distinct() maintain the order.

EDIT: I can also omit the results so we wouldn't have an assertion. What do you think?

[cigrainger]

Ah -- this will be to do with the way Polars implements the distinct call, so seeding the Elixir side won't help. For now, would you mind omitting the results and opening an issue? Thanks!

[kimjoaoun]

@cigrainger done. Now I'll open the issue :)