Port PR guidelines from sdk-typescript (#1373)

## Description

Port PR guidelines from sdk-typescript so that agents & contributors can use them.  Updated the PR.md with appropriate python examples and referenced the doc from other files.

Related files from sdk-typescript:
 
 - https://github.com/strands-agents/sdk-typescript/blob/main/docs/PR.md
 - https://github.com/strands-agents/sdk-typescript/blob/main/AGENTS.md


Co-authored-by: Mackenzie Zastrow <zastrowm@users.noreply.github.com>