Fix axe accessibility issues in demos / index.html

[mpreyskurantov]

Fixes the following axe-core found 4 violations in a standalone (nested into a WidgetsGallery iframe) demo html page / file:

1 Document should have one main landmark landmark-one-main Best practice moderate 1 2 Zooming and scaling must not be disabled meta-viewport WCAG 2 Level AA, WCAG 1.4.4 critical 1 3 Page should contain a level-one heading page-has-heading-one Best practice moderate 1 4 All page content should be contained by landmarks region Best practice moderate 1

The inclusively hidden (h1) is copied from: https://github.com/DevExpress/DevExtreme/blob/24_1/packages/devextreme/playground/jquery.html