Add documentation describing how to make your configuration

Author: ashao

doc/installation_instructions/install_notes.rst

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+.. _install-notes:
+
+General Installation Notes
+==========================
+
+SmartSim has been installed on a variety of systems and our users often have
+different build environments and toolchains. The following two sections detail
+some common situations and how to setup and modify your build environment:
+
+.. include:: install_notes/generic.rst
+
+.. include:: install_notes/nonroot-linux.rst
+
+.. include:: install_notes/offline.rst
+
+.. include:: install_notes/custom_backends.rst

doc/installation_instructions/install_notes/custom_backends.rst

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+Custom ML backends
+------------------
+
+The ML backends (Torch, ONNX Runtime, and Tensorflow) and their associated
+python packages have different versions and indices that can be supported based
+on the intended device (CPU, ROCM, CUDA-11, or CUDA-12). SmartSim stores this
+information in JSON files within the ``smartsim/_core/_install/configs/mlpackages``
+directory. If a different version or variant is needed, these can be specified
+using ``smart build --config-dir <path-to-config-dir>``. The following is the
+JSON file used for Linux with CUDA-12.
+
+.. literalinclude:: ../../../smartsim/_core/_install/configs/mlpackages/Linux64CUDA12.json
+
+The following table explains what each of the main fields are:
+
+.. list-table:: MLPackages fields
+    :widths: 25 50
+    :header-rows: 1
+
+    * - Field Name
+      - Description
+    * - name
+      - The name of the C++ frontend to the ML package itself (e.g. libtorch)
+    * - version
+      - A string used to identify the version of the library. Note that this does not have
+        an effect on the build process itself, but is used to display information
+    * - pip_index
+      - The pip index from which to install the python packages associated with this ML package
+    * - lib_source
+      - The location of the archive which contains the ML backend. If this is a URL, the file
+        will be downloaded, otherwise if this is a local path, the archive will be copied to
+        the build library and extracted
+    * - rai_patches
+      - Patch RedisAI source code with modifications needed by this ML package