feat(rule): add count constraint

[jackchuka]

This pull request introduces a new count feature to the schema structure, enabling schema authors to specify minimum and maximum occurrences for sections or headings in Markdown documents. The implementation includes schema changes, validation logic, documentation updates, and new integration tests. This provides much more flexibility for validating documents with repeated or optional sections (like step-by-step guides).

Schema and Validation Enhancements:

• Added a count field (with min and max constraints) to the StructureElement in the schema, allowing sections to be matched multiple times and validated independently. [1] [2] [3] [4] [5]
• Updated the JSON schema generator and documentation to include the new count constraint. [1] [2] [3]

Validation Logic and Engine Updates:

• Enhanced the structure validation rule to enforce count constraints, including checking for too few or too many matches and ensuring content rules apply to each match. [1] [2] [3]
• Updated the tree-building logic to support multi-match elements, correctly binding multiple nodes for repeated sections and tracking match counts and indices. [1] [2] [3] [4] [5] [6]

Documentation and Example Updates:

• Updated the README.md and example schema/tutorials to demonstrate and explain the new count feature for repeated sections, replacing previous workarounds. [1] [2] [3] [4]

Testing:

• Added comprehensive integration tests to verify correct enforcement of count constraints, including tests for valid, too few, too many, and content-rule violations within repeated sections.