Suggest that .env should be placed in .gitignore

[matthewmueller]

Right now we don't suggest that .env should be in .gitignore. I think this should be the default recommendation.

More context on the problem: https://www.notion.so/prismaio/Environment-Management-71e0177bf2cc422290aeb8733e322d46#7bee87e9b4694b36ae8fe57060e60f36

Open issues that may affect this

• Allow .env in the project root with the Prisma Schema somewhere else prisma#3720
• Adjust prisma init to write to .env instead of prisma/.env prisma#3721

[nikolasburk]

Let's add a note to the Getting Started from scratch guides: https://www.prisma.io/docs/getting-started/setup-prisma/start-from-scratch/