Make sidebar-menu-item component accessible for keyboard navigation

[aniri]

It should be possible to navigate between sidebar menu items using the tab key.

Suggestions:

• add tabindex property - https://developer.mozilla.org/en-US/docs/Web/Accessibility/Keyboard-navigable_JavaScript_widgets#Using_tabindex
• user aria attributes - https://www.w3.org/TR/wai-aria-practices/

[aniri]

please use fix for https://github.com/code4romania/taskforce-fe-components/pull/174 as reference ;)

[aniri]

closing as it was fixed