Added: - (optional) storage of user credentials in HashiCorp Vault via AI4OS Platform API ([ai4-papi](https://github.com/ai4os/ai4-papi) Changed: - rename 'frontend' to 'signup' - rename 'backend' to 'mlflow' - update README.md, .env.sample to adapt for HashiCorp Vault addition (env variables USER_CREDENTIALS_SECRETS_API, USER_CREDENTIALS_SECRETS_VO, USER_CREDENTIALS_SECRETS_PATH)