docs(manual): document sub-categories in customization, reference index, and troubleshooting

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.

docs/manual/08-components-reference/index.md

@@ -19,6 +19,10 @@ category variable.
     C5         [Logging and Capture](/08-components-reference/05-c5-logging-and-capture/) — Session logs, command duration
     C6         [Greeting & First-Run UI](/08-components-reference/06-c6-greeting-and-first-run-ui/) — Custom startup banner
 
+Each category further sub-divides into two to six sub-categories (24 in
+total) with their own `__fish_config_op_<category>_<subcategory>` toggles
+-- see that category's page for its sub-category list.
+
 ## Per-function overrides: `C0`/`always`
 
 Every guarded function or file can also carry a reserved `always/on` or