Improve “Contributing” guide with local dev steps

[jhelison]

Is your feature request related to a problem? Please describe.
CONTRIBUTING.md lacks detailed steps for setting up a dev environment (e.g., building, running tests).

Describe the solution you'd like
Add sections covering:

• Cloning repo
• Installing Go, Docker, etc.
• How to build (make build or go build)
• Running unit tests (go test ./...)
• Running e2e tests

Describe alternatives you've considered
Users figure it out from CI builds.

Additional context
Include screenshots or snippets.

[nahaktarun]

Hi @jhelison I'm interested in working on the issue. Please help to assign.