Enhancement: Add property to page folder setting to disable content creation

[estruyf]

It can happen that you have content folders from which you only want to retrieve the content, but not write any new content. To enable this, a disableCreation setting would be required.

Example: when you are migrating to a new location, but want to keep the old data in place. You can then map the folder, but tell Front Matter to not use it for content creation.