Dataframe expression explanations sould be more prominent

[savente93]

Currently I find the documentation around dataframes and dataframe expressions to be very unclear/difficult to navigate. Some examples:

1. In the documentation commands tab, there is a section labeled expression. This is about dataframe expressions specifically, and lazy data frames at that from as far as I can tell. I expected this tab to be about nu expressions specifically. This also meant that I thought a lot of documentation around dataframes such as group-by was missing, while it's actually in the expression section.
2. The difference between lazy and eager dataframes is exlained but the implications of this are mentioned only off hand. For example, the fact that you can only use dfr col in a lazy expression, but dfr as datetime can't be at all was extremely confusing to me.
3. Having three seperate pages for dfrcommands (dataframe, dataframe or lazy dataframe and expression) makes it much harder to find things and is also bad for discoverability. I for example, just thought a bunch of standard things in dataframes were not implemented because of this.

These are the issues that come to mind, I hope this will be read as constructive, as that's how I mean it, not just to criticise. These were just a few issues that caused me a lot of confusion when trying to understand them. Thanks for all the hard work!

[fdncred]

Agreed. If there aren't examples in the commands, I usually can't figure it out myself and have to ask around in our #dataframes discord channel.