make StanConnect sessions more visible - add to "Learn Stan" menu

[mitzimorris]

the StanConnect sessions are now under the "domain specific" tab.
if we break these apart, we can make "domain specific" into "Field Guides" (a more natural-sounding term),
plus a page on StanConnect sessions. This would provide a natural place to solicit StanConnect proposals.
we should capture content currently posted on Discourse.