Remove projectdoc Label

Before 1.10 projectdoc relied on the 'projectdoc' label to mark pages that are projectdoc documents. We now want to remove this visible tag and replace it with a content property. The inner workings will then check the content property instead of the tag.

For migration we will always check the content property first and then the label to determine if a page is a projectdoc document. If the 'projectdoc' label is removed we will automatically add the content property.

This is a breaking change for those that expect that projectdoc documents will have a 'projectdoc' label. For instance users who use the content-by-label macro will not find new pages.

A side effect of this change is that pages with named doctypes, that is doctypes that are not based on a blueprint, will not have the name of the doctype as a label. This is - for instance - the case with the docsections.