docs: clarify Docker and model prerequisites per sample

[however-yir]

Summary

Clarify the Docker and model prerequisites for the main samples.

Why

The root README already introduces the use cases, but a short prerequisites table makes it easier to see which samples only need Ollama and which also depend on PostgreSQL/PGVector. That helps readers choose a sample that matches their local setup.

Scope

• add a lightweight prerequisites table to README.md
• summarize the main runtime dependencies for the core use cases
• keep the guidance focused on local setup decisions rather than deployment details

Testing

• documentation-only changes
• reviewed the sample dependency notes against the current use-case READMEs and build files