docs: one documentation directory, not two (#137)

docs/ has held the README's images since the fair-launch release. I
added doc/ yesterday for the derivation spec without looking, and #108
then correctly used docs/ -- leaving the repository with both, one
letter apart, which is the kind of thing that quietly costs someone ten
minutes a year forever.

The mistake was mine, so the move is mine: derivation.md and
verify-derivation.py go to docs/, and the two README links follow.

No content changes. The verifier still reproduces the wallet from its
new path, which is the only thing here that can actually break.

0 files changed, 0 insertions, 0 deletions