Display Jupyter notebook language in document header

[mu373]

Display Jupyter notebook language in document header. Use the metadata from the notebook.

To achieve this,DocItem/Content needs to be swizzled

Conditions:

• If hide_title: don't show the document header.
• If custom_title is defined, use that as the title in document header. Ignore title or h1 tag. Remove h1 tag from Markdown, if exists.
• If custom_title nor title is defined (just has h1 tag in Markdown), use the value from h1 tag as the title.