Docs: add A11y page to coding guidelines

[jamesricky]

No description provided.

[stekalt]

Here are some points I think are missing:

• Skip links: the has to be a skip link implemented to skip the main navigation
• Focus trap for dialogs/modals: Keyboard accessibility is mentioned, but it’s not stated that focus must be trapped inside an open dialog and restored to a meaningful element when the dialog is closed.
• ARIA roles and attributes: ARIA is only mentioned in the context of labels, but other aria-attributes that are somewhat more important also should be considered, e.g. aria-label in buttons with only an icon inside, aria-expanded and aria-expanded in accordions etc. Also it should be mention to use ARIA only when necessary and always correctly.
• Color not as the only means of conveying information: WCAG requires that color alone must not be used to convey information (e.g. link styling, states, error messages).
• Zoom: content must also remain usable at 200% zoom
• Testing: Maybe it would also be good to add a short section about manual and automated testing (keyboard-only navigation, screen reader checks, color-blindness simulation, tools like getstark.co).