Expanded env vars section

[chrisj-back2work]

To cover BLITZ_PUBLIC, miscellaneous exported env vars, env, publicRuntimeConfig, and serverRuntimeConfig`.

With notes re: which are build-time vs. run-time, and how the options relate to / sometimes override each other.

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[beerose]

@chrisj-back2work here's the build issue:

You'd need to add a proper handle — use{ instead of (.