`tabIndex` in `<Page>` should be an optional prop

[rhamilto]

See https://github.com/patternfly/patternfly-react/blob/v4/packages/react-core/src/components/Page/Page.tsx#L159

This creates an issue if scrolling occurs on a child element instead of <main> where the keyboard cannot be used to scroll the scrollable element. The workaround is to add tabIndex={-1} to the scrollable element; however, it seems this should not be necessary as PatternFly should provide the option to not include tabIndex in <Page>.

[redallen]

Fixed in #4181.