Add help documentation and cleanup state tracking

Problem/Motivation

The module currently does not provide administrative help documentation for orphan cleanup functionality.

Additionally, there is no runtime state tracking for cleanup operations, making it difficult to determine when orphan cleanup was last executed.

Providing help documentation and cleanup state tracking will improve usability, transparency, and operational monitoring.

Steps to reproduce

1. Enable the Block Content Explorer module
2. Visit the Drupal help page for the module
3. Observe that no orphan cleanup documentation is available
4. Run cleanup operations
5. Observe that the module does not track the last cleanup execution state

Proposed resolution

• Implement hook_help() in block_content_explorer.module
• Add administrative help text describing orphan cleanup functionality
• Document automatic cleanup behavior and cleanup settings
• Use Drupal State API to track orphan cleanup execution timestamps
• Store the last successful cleanup runtime for cron-based cleanup operations

Remaining tasks

• Implement hook_help()
• Add cleanup documentation content
• Add State API integration for cleanup tracking
• Store and update last cleanup execution timestamps
• Test help page rendering
• Verify cleanup state values during cron execution

User interface changes

Adds module help documentation at:

admin/help/block_content_explorer

API changes

Adds integration with:

hook_help()

Data model changes

None.

This implementation uses Drupal State API for runtime tracking and does not introduce database schema changes.