Document that nested box fields in FormLayoutCustomField must hide their labels

Adds a "Nested Fields Labels" section to the `FormLayout` README explaining
that `TextField`, `TextArea`, `SelectField`, and `FileInputField` nested
inside `FormLayoutCustomField` must use `isLabelVisible={false}`, because
`FormLayout` forces horizontal layout on all descendants and a visible label
would conflict with the grid.

Updates the story and tests to follow this rule: removes prop test cases that
used visible labels on nested fields, adds `isLabelVisible={false}` to the
non-visual tests, and applies the same fix to the story component. Also adds
`Button` to the story and renames the `30px` custom label width test case to
the more representative `100px`.

Author: adamkudrna

src/components/FormLayout/README.md

@@ -330,6 +330,30 @@ and its styles may affect contained form fields through CSS cascade, don't
 forget to mirror the aforementioned properties to the contained form fields too
 as API options as such are **not** inherited.
 
+### Nested Fields Labels
+
+When placing box form fields — TextField, TextArea, SelectField, or
+FileInputField — inside a FormLayoutCustomField, we recommend to hide their
+labels with `isLabelVisible={false}`.
+
+```docoff-react-preview
+<FormLayout fieldLayout="horizontal" labelWidth="auto">
+  <FormLayoutCustomField
+    innerFieldSize="medium"
+    label="Custom field with invisible label"
+  >
+    <TextField
+      isLabelVisible={false}
+      label="This field's label is hidden"
+    />
+  </FormLayoutCustomField>
+</FormLayout>
+```
+
+ℹ️ Because FormLayout forces horizontal layout on all its descendants, the
+nested field's own label would conflict with the FormLayout grid and produce
+incorrect alignment.
+
 ### Label Alignment
 
 If you are in a situation with one or more box form fields inside your