Add categories cross-checking

We need to ensure that there's no mismatch between what stages.yml and categories.yml says, namely which stage a category is in. We also shouldn't list a listed (assumed to be active) category as a maintained one.

Additionally, the MR

1. Renames the files/methods as it no longer only checks links.
2. Fixes validation errors.

AI Summary

Enhances categories.yml validation by adding comprehensive cross-file validation between categories.yml and stages.yml to ensure data consistency.

Changes Made

CategoriesYmlChecker

• Stage-category consistency: Validates that a category's stage field matches where it's listed in stages.yml
• Unlisted category detection: Ensures all categories (except stage: unknown) are properly listed in stages.yml
• Cross-file validation: Prevents orphaned references and ensures bidirectional consistency

YamlResourceLinter

• Maintained categories validation: Ensures maintained_categories entries don't exist in categories.yml
• Category separation: Validates that categories and maintained_categories don't overlap

Tests

• Added comprehensive test coverage for new validation rules
• Updated existing tests to account for additional validation errors

Benefits

• Prevents data inconsistencies between YAML files
• Catches configuration errors during validation
• Ensures proper categorization of all features
• Maintains clean separation between regular and maintained categories

The validation now provides more consistency checking across both data files.