Added missing docs for new features

[jbednar]

Fixes #968.

• Moved spreading to the Transformation pipeline section for clarity.
• Added rescale_discrete_levels example.
• Documented 2D categorical spreading.
• Documented 3D non-categorical aggregation.
• Added antialiasing example to 3_Timeseries.ipynb .
• Added polygons docs based on Jon Mease's example notebook.

[jbednar]

@o-smirnov , if you get a chance, it would be great if you could review the category_binning and category_modulo docs finally added in this PR, adapted from #927 (comment) . Your original example was clearer, but I didn't want to use a different sample dataset, so I adapted it to the synthetic data already in use. Hopefully you won't spot any errors in my descriptions or examples!

[o-smirnov]

Thanks @jbednar, working my way through it, could I have a couple more days please?

[jbednar]

Thanks, @o-smirnov ! We'll be working on other things for at least the next day or so, but as soon as the last glitches are addressed (mostly CI) we'll be releasing. There isn't much material in here about the category stuff, so it shouldn't take long, but in any case if we have to release without you, we'll just fix it afterwards!

[jbednar]

Windows 3.7 builds are failing due to an issue with Fiona (pulled in by geopandas), but I'll go ahead and merge since that's not specifically related to this PR and I want to build the docs.

[o-smirnov]

@jbednar a bit late, but I had a look at the docs and it all looks fine to me!

[jbednar]

Thanks!