(docs) - Replace "Main Concepts" section with "Architecture" and "Authoring Exchanges" pages (#1418)

* Move "Document Caching" and "Errors" to "Basics"

* Add new "Architecture" page

* Move "Exchanges" page to "Advanced"

* Remove "Main Concepts" section

* Apply suggestions from code review
Co-authored-by: Will Golledge <35961363+wgolledge@users.noreply.github.com>
Co-authored-by: Will Golledge <35961363+wgolledge@users.noreply.github.com>