site a11y: add missing h1s

[geoffrich]

This resolves one of the Axe accessibility violations listed in #5678: Page must contain a level-one heading

Several pages on the site were missing h1 elements. Without this element, it's more difficult for screen readers to jump to the main content of the page.

In order to not affect how the page looks, I applied a class to the headings to hide them visually but still allow them to be accessed by assistive tech.

[tanhauhau]

amazing!

beside probably renaming .visually-hidden to something more meaningful, such as sr-only, or screen-reader-only?

[geoffrich]

@tanhauhau the visually-hidden name was intentional, since screen readers are not the only assistive technology that can take advantage of this class.

Bootstrap used to name their equivalent class sr-only but will be switching to visually-hidden in version 5. This class name is also used in by several a11y-focused sites in tutorials on hiding content (e.g. The A11y Project).