fix: correct ProductDocsRetrieval topic configuration in documentation and project template

[n1colasf]

Description

Fixes incorrect usage of a plain string for the topic attribute in ProductDocsRetrieval examples. The topic field requires a BaseTopic subclass reference, not a string literal like "product_docs". All documentation examples and the startproject template have been updated accordingly.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• Documentation update

Related Issues

Fixes #22

Changes Made

• Updated docs/getting-started.md: replace topic = "product_docs" with topic = ProductDocsTopic and add the corresponding import
• Updated docs/agents-tools.md: same fix across two code examples and one additional example using KnowledgeBaseTopic; updated the attribute table to reflect type[BaseTopic] as the correct type
• Updated cogsol/management/commands/startproject.py: fix the generated data/retrievals.py template to use ProductDocsTopic and include the commented import hint
• Added tests/test_content_commands.py: new test class TestStartprojectCommand covering project structure creation, correct topic class reference in the template, presence of the import hint, and failure on non-empty directories

Testing Done

• Unit tests pass locally
• Added new tests for new functionality
• Tested manually

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes