[Doc] Add documentation about <BulkUpdateFormButton> and <InputSelectorForm>

[adguernier]

BulkUpdateButton-InputSelectorForm.mp4

import * as React from 'react';
import {
    Admin,
    BooleanField,
    BooleanInput,
    Datagrid,
    DateField,
    DateInput,
    List,
    Resource,
    SimpleForm,
    TextField,
} from 'react-admin';
import { BulkUpdateFormButton } from '@react-admin/ra-form-layout';

import { dataProvider } from './dataProvider';
import { i18nProvider } from './i18nProvider';

export const App = () => (
    <Admin dataProvider={dataProvider} i18nProvider={i18nProvider}>
        <Resource name="posts" list={PostList} />
    </Admin>
);

const PostBulkUpdateButton = () => (
    <BulkUpdateFormButton>
        <SimpleForm>
            <DateInput source="published_at" />
            <BooleanInput source="is_public" />
        </SimpleForm>
    </BulkUpdateFormButton>
);

const PostList = () => (
    <List>
        <Datagrid bulkActionButtons={<PostBulkUpdateButton />}>
            <TextField source="id" />
            <TextField source="title" />
            <DateField source="published_at" />
            <BooleanField source="is_public" />
        </Datagrid>
    </List>
);