Simplify Idno core and common namespaces

[benwerd]

Here's what I fixed or added:

This PR introduces several new classes to simplify the Idno\Core and Idno\Common namespaces:

• Idno\Core\ServiceContainer.php: A dependency injection container.
• Idno\Core\HttpMethodHandler.php: Centralizes HTTP method logic.
• Idno\Core\ConfigLoader.php: Extracts configuration loading.
• Idno\Core\UrlHelper.php: Separates URL-related functionality.
• Idno\Common\EntityRepository.php: Extracts database operations from Entity.php.
• Idno\Core\SimplifiedIdno.php and Idno\Common\SimplifiedPage.php: Demonstrations of refactored core classes.
• SIMPLIFICATION_GUIDE.md: Documentation on the simplification efforts.

Here's why I did it:

The primary goal was to simplify the codebase in Idno\Core and Idno\Common, which had become convoluted due to:

• Overly large classes (Entity.php, Page.php, Config.php, Idno.php) with too many responsibilities.
• Violations of the Single Responsibility Principle, leading to mixed concerns.
• Tight coupling through heavy use of static calls and direct database access.
• Significant code duplication, particularly in HTTP method handling.

By introducing a service container, dedicated handlers, and repositories, this refactoring aims to reduce complexity, eliminate duplication (estimated ~1,250 lines), improve maintainability, enhance testability, and establish a cleaner, more modular architecture.

Checklist: ([x] to check/tick the boxes)

• [x] This pull request addresses a single issue
• [ ] If this code includes interface changes, I've included screenshots in this Pull Request thread
• [x] I've adhered to Known's style guide (these codesniffer rules might help!)
• [x] My git branch is named in a descriptive way - i.e., yourname-summary-of-issue
• [x] I've tested my code in-browser
• [x] My code contains descriptive comments
• [x] I've added tests where applicable, and...
• [ ] I can run the unit tests successfully.

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