fix: README first-run instructions reference a nonexistent keygen service

[batuhankocyigit]

fix: README's first-run key-generation instructions reference commands that don't exist

The bug

The README's very first documented step for a new node ("## 1. Node
setup") tells the user to run:

docker compose run --rm --no-deps -it keygen

docker-compose.yml has no service named keygen -- services: only
defines node. Running this exact command against the repo as-is fails
with no such service: keygen.

Two more references to the same no-longer-existing setup drifted along
with it:

• README: "the equivalent shortcuts are make init-keys and make up"
  -- there is no init-keys target in the Makefile; the actual target
  is gen-key (confirmed elsewhere in the same README, in the Operations
  table: make gen-key # first boot only: generate the validator key).
• README "Repo layout" section: docker-compose.yml # node (+ keygen, run once via \docker compose run keygen`)`.
• Makefile's own .PHONY line still lists init-keys, which doesn't
  correspond to any real target (the actual gen-key target isn't
  listed there).

This all points at the same history: the key-generation step used to be
a keygen compose service / init-keys make target, and got replaced
with a raw docker run ... new-validator-key invocation (which
docker-compose.yml already documents in a comment, and which
make gen-key already wraps) -- but the README's primary instructions
and the Makefile's .PHONY line were never updated to match.

The fix

• Replace the broken docker compose run --rm --no-deps -it keygen with
  the actual working command (matching docker-compose.yml's own comment
  and what make gen-key runs).
• Fix make init-keys → make gen-key in the shortcuts line.
• Fix the stale keygen mention in "Repo layout".
• Fix the now-inaccurate Skip `keygen` wording in the migration
  section to refer to the step generically instead of a command name that
  no longer exists.
• Fix the Makefile's .PHONY line: init-keys → gen-key.

Testing

• make -n gen-key -- dry-run output matches the command now documented
  in the README exactly.
• make help -- lists gen-key correctly (previously init-keys didn't
  show up here at all, since help parses ##-commented target lines,
  not .PHONY, so this specific symptom was invisible until someone
  actually tried make init-keys or docker compose run keygen).
• grep -rn "keygen\|init-keys" --include="*.md" --include="Makefile" --include="*.yml" . -- zero remaining matches after the fix (previously
  matched in 4 places across README.md and Makefile).

Scope

Two files, docs + one Makefile line. No changes to docker-compose.yml,
any service definitions, or actual node behavior -- this only fixes the
instructions and target name to match what already works.