docs: Add Portuguese translation to README.md

Hello Umbrel Team, I've added a Brazilian Portuguese translation to the README.md file to make the project more accessible to the Portuguese-speaking community. I used a language selector at the top with anchor links to keep the file organized and easy to navigate. The original English text remains unchanged. I believe this will help attract more users and contributors from Brazil and other Portuguese-speaking countries. Let me know if any changes are needed. Thank you!