♻️ CrawlResult model: add pydantic fields and descriptions

[gbe3hunna]

Summary

Added pydantic.Field with descriptions to the CrawlResult class to let the model be self-explanatory.
Descriptions in the codebase would have helped me during my first steps with the library.

List of files changed and why

crawl4ai/models.py - Added pydantic.Field with descriptions to the CrawlResult class to let the code be self-explanatory.

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added/updated unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes