DatePicker: fixed validation rules for Dates parameter

[stsrki]

Closes #5508

• Fixed the Dates validation.
• Add built-in validation rules for dates
• updated docs

Test:

<Validation Validator="ValidationRule.IsDateSelected<DateTime?>">
    <DatePicker TValue="DateTime?" @bind-Date="selectedDate" InputMode="DateInputMode.Date">
        <Feedback>
            <ValidationSuccess>Date is good.</ValidationSuccess>
            <ValidationError>Pick a date!</ValidationError>
        </Feedback>
    </DatePicker>
</Validation>
<Validation Validator="ValidationRule.AreDatesSelected<DateTime?>">
    <DatePicker TValue="DateTime?" @bind-Dates="selectedDates" InputMode="DateInputMode.Date" SelectionMode="DateInputSelectionMode.Multiple">
        <Feedback>
            <ValidationSuccess>Dates are good.</ValidationSuccess>
            <ValidationError>Pick dates!</ValidationError>
        </Feedback>
    </DatePicker>
</Validation>
@code {
    DateTime? selectedDate;
    IReadOnlyList<DateTime?> selectedDates;
}