Renamed the `convert-markdown` endpoint to `convert` as a
general-purpose conversion endpoint for integration with DocSpec
conversion (DOCX import), without altering its existing functionality.
In a future contribution, this endpoint will not only support conversion
from Markdown -> BlockNote -> Yjs but also directly BlockNote -> Yjs.
Signed-off-by: Stephan Meijer <me@stephanmeijer.com>
