Update FE docs on constants files for Vue i18n

What does this MR do?

Update docs about translations in vue files. For me, jumping around for ./constants or ./utils often incurs a context-switching fee. And you can access them by reference without a separate constants file:

const { MY_TEXT } = MyComponent.i18n;
expect(wrapper.find('p').text()).toMatchInterpolatedText(MY_TEXT);

Inspired by a slack thread (internal only)