[D8] Include Tour module in core

This feature proposes that we include as a demo site a sort of "getting started" book. The exact size and scope can be worked out, but I'm picturing 20-30 pages (could be more or less) that both describes and serves as an example of a simple ~~Drupal~~ Backdrop "application" that a new user could dig their teeth into. Something showing at least one content type, adding at least one field, with a vocabulary, some pictures, a view or two, maybe a form. If we want to be serious about making old-style Drupal accessible, it should also include a simple custom module and a simple custom theme, or at least mention that they exist.

A big advantage of including this bit of UX documentation with the code is that it would then be versioned with the code. 5 years from later, a developer who finds themselves suddenly in charge of a 5-year-old Backdrop 1.3 system could grab an archived distribution and know what worked at the time, instead of trying to sort through anachronistic search engine results.

(feature request/documentation)