Improve Makefile and release docs

[tbruyelle]

Current version is a copy of Gaia Makefile and is too bloated. Some targets are not required/welcomed. Here is a (to-be-completed) list of improvements:

• remove unused targets (example build-linux which breaks build reproducibility)
• remove ledger build target from all builds target (and goreleaser) except build-ledger for consistency.
• output where the binary is built ?
• revamp RELEASE_PROCESS.md
• revamp or remove create-dry-release target