[EPMTIOOPS-18112] [API Documentation] Features: Missed endpoint for get features under section

[aliaksandr5filippov]

EPMTIOOPS-18112

[API Documentation] Features: Missed endpoint for get features under section

https://jiraeu.epam.com/browse/EPMTIOOPS-18509

[S][API Documentation] Create clear documentation how to work with 'sections' on guides.test.io

Problem:

• Documentation for GET /products/{product_id}/features implied that products with sections must use the section-scoped endpoint, causing confusion about when each endpoint should be used.
• The Features API documentation page lacked an overview explaining what features are, how they are used, and how they relate to sections.

Solution:

• Clarified that GET /products/{product_id}/features returns all features regardless of whether sections are enabled, and that GET /products/{product_id}/sections/{section_id}/features is an optional alternative for retrieving features scoped to a particular section.
• Added "What are Features?" and "Features and Sections" sections with examples and guidance on when to use product-level vs section-level endpoints.

Before fix:

After fix: