doc: move failing_noop to blocks_ops.rst
Context
Closes #3915 (closed).
The Failing_noop operation is currently documented in a very heteroclite page called "Various", that is meant to disappear after dispatching its contents to appropriate places.
This MR moves the Failing_noop section to page "Blocks and operations", where it naturally belongs.
Manually testing the MR
Check the new version of page "Blocks and operations" and check that the new section fits in well.
Checklist
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