add documentation on dynamic load() paths

From the #tilt channel, someone asked:

speaking of load, is there any way to call it with a dynamic path? include doesn't seem to allow for exporting data, and load errors out unless you pass it a string literal

because they wanted to:

loading different variables into the container build based on environment (eg: dev vs staging)

Usually, when we talk to people about why starlark is designed this way, it makes sense to them (given the problems dynamic loads create in the JS module system). But we should have some documentation on this (either in the FAQ or somewhere else)

Review the existing FAQ and nearby documentation to find the best location for guidance on dynamic load() paths. Document whether load() accepts dynamic paths, explain the rationale around Starlark and JavaScript module systems, and include the dev-versus-staging use case; the issue is done when this question is answered clearly.