Suggest to add token as .env variable instead of explicit declaration

[Agathe-Brusset]

To better enforce security on our SDKs and avoid incidents, please avoid suggesting our users set explicit declaration of the API/SMTP token in the code and instead suggest setting the API key as an environment variable.

• Remove all mentions of the token added to any endpoint "ms.setToken("Your API token");"
• Suggest users to add the token as a .env variable. Name the variable MAILERSEND_API_KEY.
• Make it optional.

See Slack convo