artifacts-parameters rule should not allow URIs for the default values of artifacts parameters

That made sense in the QuickStarts repo, but since bicep compiles to a single file, using a URI is no longer necessary. That part of the rule should be removed.

Also affects documentation.

Locate the artifacts-parameters rule and its documentation; no file or test is named in the issue. Start by tracing the rule's handling of URI default values. Done means URI default values are no longer allowed and the related documentation is updated, with focused tests run if present.

Written by the indexing model from the issue text.