Add Swahili to translations staging site

The Swahili translators are waiting for the l10n process in Mozilla.

While we wait for Mozilla to provide a Swahili translation, we have started translating the Tor Browser Manual. Users will be able to use our documentation, even if they have to use the software in other language.

We need to add the manual to the staging site in https://lektor-staging.torproject.org/tor-browser-manual/translations/ to make this translation easier and better quality.