Explain how to create a custom Liquid filter in Jekyll to modify text. Describe where the Ruby code should be placed, the basic structure of the filter's Ruby code, and provide an example of how to use this custom filter within a Liquid template on a Jekyll site.
Navigate to 'jekyllrb.com'. Go to the 'Docs' section. Locate and navigate to the 'Plugins' guide. From the 'Plugins' section, find and navigate to the 'Filters' guide. Follow the link to the 'Liquid docs' for detailed information on creating custom Liquid filters. Extract information regarding filter code structure, file location, and usage in Liquid templates to answer the question.
Explain that custom filters are typically placed in Ruby files within the `_plugins` directory of the Jekyll site. Provide a Ruby code example for a custom filter (e.g., an uppercase filter), demonstrating a module and method structure (e.g., `module Jekyll::CustomFilters; def custom_filter_name(input); ... end; end;` and `Liquid::Template.register_filter`). Show how the custom filter would be used in a Liquid template (e.g., `{{ 'text' | custom_filter_name }}`).