Adds a diagram to ARCHITECTURE.md and gets rid of the BTF section,
which is orthogonal to how the library is architected. It existed
mainly to explaint why the package wasn't exported, but that is
now moot.

Make CONTRIBUTING.md shorter (do people read these things?) and
give a list of steps how to contribute a feature.

Update README.md to mention the btf package.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>