make StanConnect sessions a first-class page
This PR splits out StanConnect sessions from the Field Guides page; previously these were part of the same "Domain Specific Learning Resources" page. This makes it easier to find and therefore advertise StanConnect sessions. It also does away with the rather opaque term "Domain Specific", (assuming that the term "Field Guides" is familiar to site visitors.)
A few questions for @WardBrian and @charlesm93 - please review the new prose about StanConnect. note that the StanConnect sessions page links to the web doc that was discussed today in the Stan meeting. do we want to turn this into a Google Form? or leave as a GoogleDoc that folks can fill out as they wish.
The split seems good to me, I'll let Charles comment on the wording and logistics of the google form