[0.5] The docs io module has no submodules

[HagaiHargil]

Congrats on the new release!

Navigating to here, I can't seem to reach any submodules of the io module, such as IPC.

The symbols that are defined there are also missing from the docs as a result.

[jorgecarleitao]

Thanks a lot for noticing and reporting back! I made a hot fix for this. It also adds some labels to the features that are required to each part of the docs, e.g. https://docs.rs/arrow2/0.5.2/arrow2/compute/index.html

[HagaiHargil]

Happy to help!

I actually looked for a "good first issue" label around but couldn't find any issue to tackle.

[jorgecarleitao]

Good point, @HagaiHargil , I started to identify some of them and will continue to do so in the following days.