Add Python first build tutorial (#27824)

[ANONYMOUSZED-beep]

This commit adds a comprehensive Python first build tutorial to address the missing documentation for Python in the Getting Started guides.

Changes:

• Add site/en/start/python.md with complete tutorial covering:
  • Stage 1: Single target, single package (py_binary)
  • Stage 2: Multiple targets (py_binary + py_library)
  • Stage 3: Multiple packages with visibility
  • External dependencies with pip/PyPI
  • Testing with py_test
• Update site/en/start/_index.yaml to include Python in First build guides
• Add examples/python-tutorial/ with working example code for all stages

Fixes #27824

[bharadwaj08-one]

@ANONYMOUSZED-beep Could you please take a look at the failing checks?