Update "operators overview" page to highlight SparsePauliOp and split in-depth Operator coverage to separate page (#1517)

Closes #1440 

@kevinsung, here's what I have so far in terms of content to include. I
think it might be worth mentioning the overloaded operators for the
`SparsePauliOp` class and demonstrate some of the algebra that can be
done.

---------

Co-authored-by: Kevin J. Sung <kevjsung@umich.edu>

docs/build/_toc.json

@@ -46,10 +46,14 @@
           "title": "Operators module overview",
           "url": "/build/operators-overview"
         },
-	{
-	  "title": "Specify observables in the Pauli basis",
-	  "url": "/build/specify-observables-pauli"
-	}
+        {
+          "title": "Specify observables in the Pauli basis",
+          "url": "/build/specify-observables-pauli"
+        },
+        {
+          "title": "The Operator class",
+          "url": "/build/operator-class"
+        }
       ]
     },
     {

qiskit_bot.yaml

@@ -55,6 +55,8 @@ notifications:
     - "@Cryoris"
   "docs/build/operators_overview":
     - "`@mtreinish`"
+  "docs/build/operator-class":
+    - "`@mtreinish`"
   "docs/build/pulse":
     - "`@nkanazawa1989`"
     - "@abbycross"

scripts/nb-tester/notebooks.toml

@@ -34,6 +34,7 @@ notebooks_normal_test = [
     "docs/build/circuit-library.ipynb",
     "docs/build/circuit-visualization.ipynb",
     "docs/build/classical-feedforward-and-control-flow.ipynb",
+    "docs/build/operator-class.ipynb",
     "docs/build/operators-overview.ipynb",
     "docs/build/pulse.ipynb",
     "docs/build/save-circuits.ipynb",