Respect content_includes_title frontmatter flag

[fwkoch]

If MyST knows the title is duplicated in the frontmatter and in the content (e.g. if the title is plucked from a heading that is not at the top of the content, it will not be removed), MyST adds a content_includes_title: true flag to the frontmatter. This gives the author more flexibility about where to add the title (e.g. after an introductory statement or something - for example, https://github.com/ue12-p24/intro/blob/f4799080694aec8524c5a243f59e6a7d1c0495e6/notebooks/0-00-presentation-ue12-ue22-nb.md).

In these cases, myst-theme should probably not display the title in the title block. This does call into question what happens with the other stuff in the title block (e.g. authors, affiliations, etc).