Document how to create Templates and Tutorials

[fflorent]

It can be hard to understand how to make templates and (especially) tutorials. Creating a support page would help users to create their own.

In this support page, it would be neat to explain how:

• [ ] To create the template organization (maybe rather document it in the "self-managed" page);
• [ ] How tutorials and templates work, what they are meant for;
• [ ] How to create a document from scratch and convert it (which would normally be done in https://github.com/gristlabs/grist-core/issues/1015) or from an existing document (which is already explained here);
• [ ] How to create a tutorial;

Related issues:

• https://github.com/gristlabs/grist-core/issues/1015
• https://github.com/gristlabs/grist-help/issues/374