Create new prototyping section | Address some comments | Small clarifications

docs/Prototyping/index.rst

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+Prototyping Flow
+================
+
+Chipyard supports FPGA prototyping for local FPGAs supported by `fpga-shells <https://github.com/sifive/fpga-shells>`__.
+This includes popular FPGAs such as the Xilinx VCU118 and the Xilinx Arty board.
+FPGA prototyping allows for orders-of-magnitude faster speeds than software RTL simulators at the cost of slower compile times and less design introspection.
+
+.. Note:: While ``fpga-shells`` also supports Xilinx VC707 and some MicroSemi PolarFire boards, currently only the VCU118 and Arty boards are explicitly supported in Chipyard.
+    However, using the VCU118/Arty examples would be useful to see how to implement VC707/PolarFire support.
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Prototyping Flow:
+
+   General
+   VCU118
+   Arty