fix(docs): restore missing code block in pt-BR first-flow guide (#5780)

* fix(docs): restore missing code block in pt-BR first-flow guide

The pt-BR translation of the 'Build Your First Flow' guide had a
placeholder comment '# [CÓDIGO NÃO TRADUZIDO, MANTER COMO ESTÁ]'
instead of the actual Python code in Step 5. This restores the full
main.py code block from the English source, matching the original
since code should not be translated.

* Translate code comments to pt-BR in first-flow guide

Code comments in the tutorial should be in Portuguese for the pt-BR
audience, since they are part of the guide's educational content.