Docs/add custom fields documentation

[ghadabezine]

Description

This PR adds documentation explaining best practices for creating custom field components in react-admin, with a specific focus on using defaultProps for labels. This addresses issue #10684.

Changes

• Created new documentation file docs/CustomFields.md
• Added comprehensive guide on creating custom field components
• Included detailed examples of using defaultProps for labels
• Added best practices for field component development
• Provided usage examples with different label configurations

Related Issues

Closes #10684

Documentation

The new documentation covers:

1. Basic structure of custom field components
2. Using defaultProps for labels
3. Best practices for field component development
4. Complete example with all best practices
5. Usage examples with different configurations

The documentation emphasizes the importance of using defaultProps for labels to ensure consistency and maintainability across custom field components.

[djhi]

Thanks for the PR. Why did you update SelectArrayInput?

[slax57]

Also, I'm not sure this PR goes in the right direction. To me, fzaninotto's answer rather suggested we want to move away from defaultProps, as they are deprecated and will be removed.

At the very best we can clarify why they are still needed in some cases, and make it clear they are deprecated so alternatives should be considered, but IMHO we should not say adding defaultProps is recommended.

Also, I believe we should update the existing doc section rather than creating a new one (which is not referenced in the navigation menu btw).

Lastly, I believe the addLabel prop is no longer relevant in RA v5.

[slax57]

see #10738 (comment)