Insert zettel title as top heading if not done

To get consistently looking cards, zettel's title will be inserted in <h1> tag in case it is missing in zettel's body.

First, the title from yaml frontmatter (title: key) will be used.

If this is missing too, then the filename will be used.

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def greet(name: str) -> str:
    """Return a greeting."""
    return f"Hello, {name}!"

print(greet("Zettelkasten"))