doc: generate documentation for lib.customisation

Changed files
+1
doc
+1
doc/default.nix
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{ name = "sources"; description = "source filtering functions"; }
{ name = "cli"; description = "command-line serialization functions"; }
{ name = "gvariant"; description = "GVariant formatted string serialization functions"; }
];
};
···
{ name = "sources"; description = "source filtering functions"; }
{ name = "cli"; description = "command-line serialization functions"; }
{ name = "gvariant"; description = "GVariant formatted string serialization functions"; }
+
{ name = "customisation"; description = "Functions to customise (derivation-related) functions, derivatons, or attribute sets"; }
];
};