Explain theme creation for the manager

[fkaminski]

The export & import of Contao themes in the backend (via .cto) is described here:
https://docs.contao.org/manual/en/layout/theme-manager/manage-themes/

You can now also install themes (incl. database) via the Contao Manager during the initial installation -
see Contao Demo: https://github.com/contao/contao-demo/tags
However, these differ in terms of structure (composer.json) etc..

A corresponding guide explaining the differences and how to create them for use in the Manager could be helpful.

If desired, please assign to myself.