docs: Add Custom Rules info

[coliff]

This pull request introduces comprehensive support for custom HTMLHint rules, including examples, documentation, and CLI enhancements. The changes aim to make it easier for users to create, configure, and use custom rules to extend HTMLHint's functionality.

Custom Rule Examples and Implementation:

• examples/custom-rules/example-rule.js: Added an example custom rule that checks for accessibility attributes on images and validates class names against a configurable pattern. The rule demonstrates integration with the HTML parser and reporter.
• examples/custom-rules/README.md: Documented the example custom rule, including its purpose, usage, configuration, and testing instructions.

Documentation Updates:

• website/src/content/docs/usage/custom-rules.mdx: Created a detailed guide on creating, using, and organizing custom rules, including best practices, examples, and troubleshooting tips.
• website/src/content/docs/usage/cli.md: Expanded the CLI documentation for the --rulesdir flag, showing how to load custom rules from files or directories.
• website/src/content/docs/usage/options.md: Added documentation for the rulesdir option, with usage examples and details on directory loading behavior.

[gemini-code-assist]

Code Review

This pull request adds comprehensive documentation for creating and using custom rules in HTMLHint, including a new documentation page, examples, and updates to CLI and options documentation. The documentation is well-structured and detailed.

My review focuses on improving the correctness and robustness of the provided code examples. The key suggestions are:

• Using arrow functions in event listeners to ensure the correct this context.
• Handling rule options more safely, especially for falsy values like 0.
• Improving performance and robustness in the example rule by caching RegExp objects and handling potential errors.

These changes will ensure the examples follow best practices and are more reliable for users to copy and adapt.

[gemini-code-assist]

The line const maxAttrs = options || 5; is not robust for handling options. If a user configures "max-attributes": 0 in their .htmlhintrc, options will be 0, and 0 || 5 will evaluate to 5. This is likely not the user's intent.

A safer way to provide a default value is to check the type of options or check specifically for undefined.

      const maxAttrs = typeof options === 'number' ? options : 5; // Default to 5 if no options provided