Document more parts of the library

With #1017 merged and a working copy hosted at https://ebpf-go.dev, I've been asking existing contributors to write on specific topics I know them to be familiar with. This issue acts as an index of topics I'd like to cover before giving the documentation a wider announcement (Golang weekly, Slack, contacting blog authors, ebpf.io and eBPF weekly), since currently there's quite a few pages that are stubbed out that really should be covered.

Topics we're looking for help for:

• kfuncs
• Maps: declaring and using
• Attaching Programs
• Testing @NIKOPACK
• Feature detection @rgo3
• Portable eBPF @lmb
• bpf2go @lmb
• BTF