add information to the website about how to get started with gridsync

[exarkun]

The gridsync docs themselves are pretty good and we can probably re-use a lot of the content. We should have something that is narrowly focused on how to go from the wormhole-based post-signup page to having a gridsync talking to an LAE S4 subscription's storage server.

There's also the possibility of non-wormhole-based gridsync use at some point but this issue is only for the wormhole-based process.