Sub-categories were previously undocumented outside README and the
per-category reference pages. Add a paragraph to 07-customization.md (the
canonical opinionated-components page) explaining the sub-category cascade,
a short note to the 08-components-reference index pointing readers at each
category's sub-category list, and a troubleshooting note that
__fish_config_op_<category>_<subcategory> variables follow the same
check/reset pattern as category variables.
rootiest
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