arrow

ARROW-4304: [Rust] Enhance documentation for arrow

Hi all,

I hope I don't bother you too much with this.
I recently started using the Rust implementation of Apache Arrow and it's going well so far! However, I noticed that the documentation is a bit sparse.

I then dug a little bit around in Jira and noticed there are a bunch of issues targeting enhancement of documentation, most notably [ARROW-4304](https://issues.apache.org/jira/browse/ARROW-4304), [ARROW-4683](https://issues.apache.org/jira/browse/ARROW-4683) and [ARROW-4927](https://issues.apache.org/jira/browse/ARROW-4927).

I then started documenting a few things and adding working doctests to it. I also updated some external links that were broken.

It's far from complete but I guess it's a decent start for a wonderful documentation of this crate :blush:

I am happy to discuss what I've written :innocent:

- [x] tests passing (including doctests)
- [x] code formatting passes
- [x] link check (only for external links)

Closes #6828 from Jibbow/arrow-doc

Authored-by: Andreas Zimmerer <andreas.zimmerer@tum.de>
Signed-off-by: Andy Grove <andygrove73@gmail.com>