Add detailed verifier docs + guide to verifier log reading

[dylandreimerink]

For most developers the verifier is the main obstacle to content with when it comes to writing eBPF programs. We already have a generic concept page which doesn't go into details. It would be valuable to make some more detailed documentation surrounding the inner workings of the verifier. The verifier does output a log, but its hard to read, especially newcomers.

[MahnurA]

Also add information about frequent verifier complaints and what those mean.