Add footer to language_switcher

What does this MR do and why?

This MR adds a footer with a link "Help translate to your language" to the upcoming language_switcher component (shown in the Sign-in page).

This is to address this other MR's thread of communicating a message that the languages in the dropdown are not all of the languages a

Screenshots or screen recordings

• The "Help translate to your language" link navigates to this docs page of the GitLab instance.

How to set up and validate locally

1. Enable the preferred_language_switcher feature flag in the Rails console (rails c)

   Feature.enable(:preferred_language_switcher)

2. Visit the login page of the GDK instance

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.