Doc section about projectors.

docs/advanced_tutorials/index.md

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+In this section, advanced programming concepts and implementations in Qadence are examplified.

docs/advanced_tutorials/projectors.md

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+This section introduces the `ProjectorBlock` (defined [here](qadence.blocks.primitive.ProjectorBlock)) as an implementation for the the quantum mechanical projection operation. It evaluates to the outer product of a ket and a bra expressed as bitstrings.
+
+!!! warning
+    Projectors lead to non-unitary computations.
+
+
+```python exec="on" source="material-block" session="noise" result="json"
+from qadence.blocks import block_to_tensor
+from qadence.operations import Projector  # Projector as an operation.
+
+
+projector_block = Projector(ket="1", bra="1", qubit_support=0)
+
+# As any block, the matrix representation can be retrieved.
+projector_matrix = block_to_tensor(projector_block)
+
+print(f"projector matrix = {projector_matrix}") # markdown-exec: hide
+```
+
+Other standard operations can be expressed as projectors: for instance projector controlled-unitary gates or the number operator $N$.