Installation and sample usage

**Describe the feature you'd like**

At this stage it's unclear how to actually use the `arnold-usd` repository and how to adopt or even test the functionality.

**Describe the solution you'd like**

1. It would be good to briefly elaborate what the correct environment should be after building, e.g. what should be added to `PATH` or maybe to some Arnold-related environment variables to have the procedurals/plug-ins found. It would be good to have a simple example (potentially per OS).

2. With `arnold_to_usd` as a helper utility it could be good to have a *Getting Started* tutorial that describes taking an [example Arnold `.ass` file](https://docs.arnoldrenderer.com/display/A5ARP/.ass+File+Examples) and take it through the executable.

---

For example:

1. Set up your environment
```bat
:: Psuedocode (random example)
set PATH={root}/bin;%PATH%
set PATH={root}/lib;%PATH%
set PATH={root}/procedural;%PATH%
set ARNOLD_PLUGIN_PATH={root}/plugin
```

2. Download [an example `.ass` file from Arnoldpedia](https://docs.arnoldrenderer.com/display/A5ARP/.ass+File+Examples)

3. Run `arnold_to_usd` to convert it to a usd stage.
```
arnold_to_usd /path/to/input.ass /path/to/output.usd
```

4. Add the procedural to an Arnold scene and load the created USD scene. (or can we render it directly?)

5. (Optional) Show how one could layer a change into the `.usd` file and influence the Arnold render using a secondary `.usd` file.
```
#usda 1.0

over "Camera"
{
float[] arnold:focus_distance = [50.0]
}
```

Start with the `arnold_to_usd` helper and the proposed command using an example Arnold `.ass` file. Document the post-build environment setup, including plugin discovery, then add a Getting Started walkthrough from downloading the example through producing and loading the USD stage. Done means a newcomer can follow the environment and conversion steps, with platform-specific differences noted where needed.

Written by the indexing model from the issue text.