Python: [codex] Add hosting samples overview README

[whenpoem]

Summary

This docs-only PR adds a top-level README for python/samples/04-hosting/ so beginners can quickly understand the available hosting approaches and jump to the right sample set.

Closes #5403.

What Changed

• added python/samples/04-hosting/README.md
• listed the four hosting options (a2a, azure_functions, durabletask, and foundry-hosted-agents)
• added brief guidance on when to choose each hosting model
• documented only the common prerequisites shared across hosting samples
• linked readers to each subdirectory README for hosting-specific setup

Impact

This improves navigation for new users exploring Python hosting samples without changing sample code or runtime behavior.

Validation

• verified all four relative README links point to existing files
• confirmed the change is docs-only and adds a single new file
• confirmed the common prerequisites section does not treat Azurite, Azure Functions Core Tools, or Docker as universal requirements

[whenpoem]

@microsoft-github-policy-service agree