docs: improve environment setup and add .env.example files

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Issue Description ✍️

Added .env.example to blockchain and client

Made environment setup clearer

Explained dummy vs real keys

Updated README accordingly

Improved new contributor onboarding

### Record

- [x] I have synced all my node versions as mentioned in the project
- [x] I am using the same version of npm as is the project
- [x] My current branch is in sync with the development branch
- [x] I want to work on this issue