Execution span docs (Qiskit#1918)

* Make execution_span a composite module and add docs

* add to sphinx

* lint

* add module docstrings

docs/apidocs/execution_span.rst

@@ -0,0 +1,4 @@
+.. automodule:: qiskit_ibm_runtime.execution_span
+   :no-members:
+   :no-inherited-members:
+   :no-special-members:

docs/apidocs/index.rst

@@ -14,3 +14,4 @@ qiskit-ibm-runtime API reference
    transpiler
    qiskit_ibm_runtime.transpiler.passes.scheduling
    fake_provider
+   execution_span

qiskit_ibm_runtime/execution_span/__init__.py

@@ -0,0 +1,41 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2021.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+"""
+==========================================================
+Execution Spans (:mod:`qiskit_ibm_runtime.execution_span`)
+==========================================================
+
+.. currentmodule:: qiskit_ibm_runtime.execution_span
+
+Overview
+========
+
+An :class:`~.ExecutionSpans` class instance is an iterable of :class:`~.ExecutionSpan`\\s, where 
+each iterand gives timing information about a chunk of data. Execution spans are returned as part 
+of the metadata of a primitive job result.
+
+Classes
+=======
+
+.. autosummary::
+    :toctree: ../stubs/
+
+    ExecutionSpan
+    ExecutionSpans
+    ShapeType
+    SliceSpan
+"""
+
+from .execution_span import ExecutionSpan, ShapeType
+from .execution_spans import ExecutionSpans
+from .slice_span import SliceSpan