Document more parts of the library

[ti-mo]

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 @markpash
• [ ] Attaching Programs
• [ ] Testing
• [x] Feature detection @rgo3
• [ ] Portable eBPF @lmb
• [ ] bpf2go @lmb
• [ ] BTF

[lmb]

I think I'd prefer starting with portable ebpf if that is ok?

[lmb]

Should we take out the current WIP pages and update links everywhere, for now?