Remove the path parameter from the article creation API

Edit: a use case has come up for this: if we can find an existing article that the user is trying to update, we might be able to determine that it does not need to be converted in the first place: skip re-render from API if article was unchanged. But then of course we can't render the article to find its ID, as the hole point is to skip that render in the first place.

We likely want to get rid of the path parameter, and instead determine IDs fully from more "in-band" things like {id} and {scope}.

Both {scope} for subdirs and {id} for custom id basename !== from title should already be working, we just haven't setup ourbigbook CLI to inject {id} based on file path I think.

{scope} is however not really usable in general on the same source tree of cirosantilli.github.io due to github.com/ourbigbook/ourbigbook/issues/284.

This would forbid some constructs that are currently possible locally, e.g. scopes that are not children such as:

parent.bigb

parent2.bigb

child/subdir.bigb

but that is fine, it is saner if we enforce scopes to match the tree article tree hierarchy.