[Doc] Add <AutoSave> documentation

[fzaninotto]

import { AutoSave } from '@react-admin/ra-form-layout';
import { Edit, SimpleForm, TextInput, DateInput, SelectInput, Toolbar } from 'react-admin';

const AutoSaveToolbar = () => (
    <Toolbar>
        <AutoSave />
    </Toolbar>
);

const PersonEdit = () => (
    <Edit mutationMode="optimistic">
        <SimpleForm
            resetOptions={{ keepDirtyValues: true }}
            toolbar={AutoSaveToolbar}
        >
            <TextInput source="first_name" />
            <TextInput source="last_name" />
            <DateInput source="dob" />
            <SelectInput source="sex" choices={[
                { id: 'male', name: 'Male' },
                { id: 'female', name: 'Female' },
            ]}/>
        </SimpleForm>
    </Edit>
);

• Add AutoSave Chapter
• Mention AutoSave in Reference and sidebar navigation
• Mention AutoSave in related chapters
• (Bonus) Improve existing form documentation

[slax57]

What a titanic job! 👏
(and a never-ending review 😅 )

[fzaninotto]

Review applied