[Accessibility][Meta] Headings in examples and on pages should be accessible

[citelao]

Which version of the app?

WinUI 3 Gallery

Description

There are visual headings all over the gallery, but they are typically not accessible as headings.

We should:

• Add AutomationProperties.HeadingLevel to each heading.
• Make a Style to allow folks to easily add accessible headings in code.

Screenshots

No response

Windows version

No response

Additional context

No response

[citelao]

Partially fixed in #1223