SG Content - Sync Gateway Docs Refresh

This ticket is being re-purposed and upcycled to cover a wider review and refresh of the Sync Gateway documentation in line with:

• Keeping the design, navigation, and topic content simple and succinct
• The product's fundamental function: to Sync Securely - locally, centrally, and at the edge 

Target user feedback: As a prospective Couchbase user/customer I felt the documentation clearly explained what Sync Gateway is, the features it provides, how those features can be applied to various use-cases, and what we need to do to make it work for us! 

The existing ticket tasks (around concepts) generally still apply and will be incorporated as appropriate (DOCS-5890, 5891, 5893)

 
Original Description:

Overwhelming number of users (especially community users) as well as customers and our own field teams are feeling the pain of dealing with a very complex system with limited or confusing documentation. 

Improvements on description of foundational concepts would be critical for the on boarding of new users

This ticket is an umbrella ticket to capture every such potential enhancement

(CC Amarantha Kulkarni)