[FND-3] Release 7.0.0

[lucassus]

This pull request updates the documentation for the Maxio Advanced Billing SDK, including enhancements to readability, version upgrades, and new additions to the SDK's utility classes. It also introduces structural improvements to the documentation hierarchy and corrects some formatting inconsistencies.

Documentation Updates and Improvements:

• Updated README.md to improve readability, simplify steps for making API calls, and add links to relevant sections.
• Upgraded SDK version references from 6.1.0 to 7.0.0 across all documentation, including dependency installation, API client initialization, and controller links. [1] [2]

New Additions:

• Added a new ApiHelper utility class documentation, detailing methods for JSON serialization and deserialization.

Structural and Formatting Improvements:

• Standardized relative links to documentation files for better navigation (e.g., updated links in doc/api-exception.md and doc/client.md). [1] [2]
• Introduced a new "SDK Infrastructure" section in README.md to organize configuration, HTTP, and utility class documentation.

Minor Corrections:

• Fixed formatting inconsistencies in parameter descriptions (e.g., added extra line breaks for clarity in doc/controllers/api-exports.md).
• Renamed "ApiException Class" to "ApiException" for consistency.