Document "best practices" for variable naming (and update examples to use it)

[bpkroth]

As discussed in https://github.com/microsoft/MLOS/pull/960/files/393e0388b3f56e234277172094a28a5f76c8e9bd#r2006409736

since we have a single namespace for variables in MLOS configs, but we want them to be reusable, we should

• document best practices around how to do that without clashes
• update our examples to follow