Document the special GristTourDoc for a tour of the document

[fflorent]

It is possible for document owners (maybe editors too?) to create a tour of the document, so the visitors can understand how to use the document they discover.

Such thing is very powerful though not documented.

I propose to describe:

• The use case for a tour;
• How to create step by step the GristTourDoc special table;
• How it finally renders for visitors;

A post in the French Grist community exists, and may be helpful [in French]: http://forum.grist.libre.sh/t/gristdoctour-creer-un-tour-de-visite-decouverte-de-votre-projet/68?u=florent.fayolle