Add max execution time to faq (Qiskit/qiskit-ibm-runtime#294)

* Add maximum execution time to faq

* Adjust headings

* Make heading a question

* change 's to is in the first FAQ

* Fix title overline too short

* split faqs into different files

* Fix breadcrumbs

Co-authored-by: Rathish Cholarajan <[email protected]>

runtime-docs/faqs/max_execution_time.rst

@@ -0,0 +1,41 @@
+.. _faqs/max_execution_time:
+
+============================================================
+What is the maximum execution time for a Qiskit Runtime job?
+============================================================
+
+To ensure fairness, there is a maximum execution time for each Qiskit Runtime job. If
+a job exceeds this time limit, it is forcibly cancelled. This is represented in the job
+status as `Cancelled - Ran too long`. The maximum execution time is the
+smaller of 1) the system limit and 2) the ``max_execution_time`` defined by the program.
+The system limit is defined below:
+
+Qiskit Runtime on IBM Cloud
+---------------------------
+
+The system limit on the job execution time is 3 hours for a job running on a simulator
+and 8 hours for a job running on a physical system.
+
+Qiskit Runtime on IBM Quantum
+-----------------------------
+
+The system limit on the job execution time is
+
++------------------+--------------+-----------+--------------+-----------+
+|                  | Public Program           | Private Program          |
++==================+==============+===========+==============+===========+
+|                  | Premium User | Open User | Premium User | Open User |
++------------------+--------------+-----------+--------------+-----------+
+| Simulated Device | 3h           | 1h        | 3h           |1h         |
++------------------+--------------+-----------+--------------+-----------+
+| Real Device      | 8h           | 4h        | 8h           |2h         |
++------------------+--------------+-----------+--------------+-----------+
+
+Note that a *premium user* here means a user who has access to backends in providers other than ibm-q/open/main.
+
+
+Other Limitations
+-----------------
+
+- Programs cannot exceed 750KB in size.
+- Inputs to jobs cannot exceed 64MB in size.

runtime-docs/faqs/open_source_vs_ibm_cloud_primitives.rst

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+.. _faqs/open_source_vs_ibm_cloud_primitives:
+
+=================================================================================================
+What is the difference between the open source primitives and primitives available via IBM Cloud?
+=================================================================================================
+
+The open source primitive contains the base classes (to define interfaces) and a reference implementation.
+The Qiskit Runtime primitive is planned to provide more sophisticated implementation (such as with error
+mitigation) as runtime service.