Add guidance for when interfaces are slow to load, or fail to load

[nathandyer]

Describe the change

This would be useful for both the source and journalist guides.

For those who are coming to Tor for the first time, the expectation of general sluggishness or slower loading times might not be established. It may appear that something is not working correctly, or that the browser/interface is "frozen."

We should note what to do when this happens (refresh, or "New Tor Circuit for This Site"), and explain more about why it happens.