Problem: Currently, transaction and storage fees are described in
michelsonExploration.md, and gas is described in a separate
gasConsumption.md. It is tricky to find information because the
documentation is not cohesive: info on two closely related concepts
is scattered across the repo, and there are no links connecting
the docs.

Solution: Move fees description to a separate document, link to
this document and to the gas document from michelsonExploration.md,
link to the gas document from the fees document.