docs: add READMEs for instrumentation modules

[axw]

We should add READMEs for each instrumentation module, providing basic usage docs which will show up on pkg.go.dev. When we do this, we may want to use the content to generate docs for elastic.co.

[bmorelli25]

👋 Going through old issues today. I clicked through a few of the modules on this list https://pkg.go.dev/go.elastic.co/apm/module, and it seems like they all have readmes. Is this issue still relevant?

[axw]

Still somewhat relevant I think. It seems pkg.go.dev no longer shows that warning, but shows the basic package doc comment. I'd like to have some basic documentation about usage show up on that site, as it's pretty common avenue for Go devs to get their docs.