Improve documentation

[danielocfb]

We got some feedback that the documentation we have is generally not the greatest.

• [ ] the belief is that more examples showing best practices and common patterns would go a long way making the library more user-friendly
  • we should also highlight the preferred way of going about things
• [ ] we have also heard that using bpf_objects is somewhat difficult (that may be an API issue or documentation; needs to be looked at)
• [ ] on a more mechanical level, let's make sure to remove all #[allow(missing_docs)] instances

[noboruma]

Hello @danielocfb As a newcomer, the main pain point for me was to find a working example that I could play with. I have put the XDP example (from bootstrap repo) into a cargo template repository: https://github.com/noboruma/libbpf-rs-template I feel this could be a great way to get started quickly with the library. It has room for improvements - it would be nice to support other type of probing for instance. Let me know if that's something you guys would like to expand.