Added software section. Marshal is populated (mostly points to Marshal's own readthedocs). Spike is a stub.

docs/Software/index.rst

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+Target Software
+==================================
+
+Chipyard includes tools for developing target software workloads. The primary
+tool is FireMarshal, which manages workload descriptions and generates binaries
+and disk images to run on your target designs. Workloads can be bare-metal, or
+be based on standard Linux distributions. Users can customize every part of the
+build process, including providing custom kernels (if needed by the hardware).
+
+FireMarshal can also run your workloads on high-performance functional
+simulators like Spike and Qemu. Spike is easily customized and serves as the
+official RISC-V ISA reference implementation. Qemu is a high-performance
+functional simulator that can run nearly as fast as native code, but can be
+challenging to modify.
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Contents:
+
+   FireMarshal
+   Spike