Improve petgraph docs for new users

[JosiahParry]

Summary

I'm trying to pickup petgraph for spatial statistics however I'm finding it hard to get going due to a lack of examples. It would be great if there were more worked examples and documentation on getting started and the intended use patterns for the library.

Motivation

Adding this eases adoption.

Details

• What code needs to change? New traits? Which modules?

No code needs to change. Additional doc in the source files would be needed.

• Is this a new graph algorithm? Include links to papers, Wikipedia, or other implementations of this algorithm that exist.

No.

• Are you willing to implement this yourself? Mentor someone else and help them implement it?

Yes, if provided with guidance.

[highway900]

Reading the tests is a good way to better understand the crate. Though I do agree there is a need for clear examples. Some examples of what you would like to see might help :)