docs: add portuguese (pt_BR) documentation

[JulioPeixoto]

Description

This PR adds initial Portuguese (Brazilian Portuguese) documentation for the ADK project, making it more accessible to Portuguese-speaking developers. The documentation includes:

• Basic setup and installation instructions
• Core concepts and features
• Contribution guidelines
• Authentication setup
• Code examples with latest model versions

Changes include

• Set up Sphinx configuration for pt_BR
• Create basic documentation structure
• Add translated content for:
  • Introduction (introducao.rst)
  • Installation guide (instalacao.rst)
  • Basic concepts (conceitos_basicos.rst)
  • Contributing guide (contribuindo.rst)
• Configure build system for multi-language support
• Add examples using latest ADK features

[ctarnold]

As a Spanish speaker who can read Portuguese, the docs seem well written and translated. Style consideration, words like "pipeline, issue, branch, Bugs, pull request" are left in English, but I agree with this choice. Translating these terms could be possible but might come at the cost of exact communication. Native Portuguese speakers can of course chime in who would have a more educated opinion. Offering my feedback as there may not be any current contributors who read Portuguese (at time of writing no contributors have location Brazil or Portugal on their profiles).

[hangfei]

@polong-lin (our doc lead) to help review this change and our multi-lingual support.

[JulioPeixoto]

i've just discovered the adk-docs repo, it would be nice to have some reference to it here.