[Bridges] Add dnstt instructions

As volunteers will be able to run bridges with dnstt pluggable transport, we will need to update bridges documentation. Since it's a go binary, maybe we should have one page with instructions instead of doing platform specific.

As dnstt bridges is still under development, we should follow this ticket timeline: tpo/anti-censorship/pluggable-transports/trac#40001 (comment 2788292)