docs: Add Ibis to alternatives.md

[NickCrews]

I think Ibis and Polars aim to solve a lot of the same problems, and do so with similar philosophy, so I think there should be a lot of overlap in interest.

[alexander-beedie]

Hey @NickCrews, thanks for your interest in polars - I have actually previously contributed to Ibis for their v6.0 release, so I'm not unfamiliar with the project (and I wrote an equivalent library years ago inside JPMorgan) ;)

However, I don't actually think it's a good match for our "Alternatives" entry; all of the other comparisons there are engines in their own right, whereas Ibis is not - adding it here would be a bit like comparing SQLAlchemy to PostgreSQL. They're playing in the same space, but they aren't directly comparable. I couldn't swap my usage of Polars for Ibis, for example, without also having something like DuckDB installed for Ibis to use, at which point the real comparison would be with DuckDB (which is in our "Alternatives" section).

Will close this out for that reason, but thanks for taking the time to make a contribution! ✌️