The list in FURTHER_READINGS.rst hasn't been updated in nearly four
years, and doesn't reflect well the activity (in terms of publication)
around the project. It makes no sense keeping it at the root of the
directory. Following a discussion with Joe, we agreed that updating this
list would take time and would lead to the same issue in a few years
from now, so it's probably best to point to external, more relevant
resources or, to be more precise, lists of resources.

So we remove this document, and in the documentation, we point instead
to the list of links on ebpf.io, and the blog and case studies on
cilium.io, where we hope readers will find what they're looking for; at
least, they should find more recent reading materials.

Signed-off-by: Quentin Monnet <qmo@qmon.net>