Re #21181. Add MaskSpectra Sphinx documentation.

docs/source/algorithms/MaskSpectra-v1.rst

+
+.. algorithm::
+
+.. summary::
+
+.. alias::
+
+.. properties::
+
+Description
+-----------
+
+Mask (zero) spectra and the underlying detectors in a workspace.
+If a spectrum corresponds to more than one detector, all detectors are masked.
+
+Usage
+-----
+
+**Example - MaskSpectra**
+
+.. testcode:: MaskSpectraExample
+
+  ws = CreateWorkspace(OutputWorkspace='ws', DataX='1,1', DataY='2,2', NSpec=2)
+  masked = MaskSpectra(InputWorkspace=ws, InputWorkspaceIndexType='WorkspaceIndex', InputWorkspaceIndexSet='1')
+  print(ws.readY(0), masked.readY(0))
+  print(ws.readY(1), masked.readY(1))
+
+Output:
+
+.. testoutput:: MaskSpectraExample
+
+  (array([ 2.]), array([ 2.]))
+  (array([ 2.]), array([ 0.]))
+
+.. categories::
+
+.. sourcelink::
+