Documentation TODOs

[willfindlay]

This is a general-purpose tracking issue for several areas of improvement in Aya's documentation. I'm planning on spending some cycles on some of these over the next couple of weeks. If anyone has any comments/suggestions on something to add or change here, do let me know.

Short-term goals (to help new contributors):

• Add an ARCHITECTURE.md file detailing the architecture of the project
• Add a description of steps needed to 1) implement a new program type; and 2) implement a new map type to CONTRIBUTING.md

Long-term goals (to help library users):

• Document every public-facing part of the userspace API
• Document every public-facing part of aya-bpf
• It probably also would make sense to #![deny(missing_docs)] at that point