#1924 docs: CONFIGURATION.md reference documentation for all flags and environment variables

Description

## CONFIGURATION.md reference documentation for all flags and environment variables

## Summary
I thought it might be helpful to have a configuration reference document that included every flag and environment variable, since currently they're scattered all around the other docs, and many are not documented at all. This has already helped me find an environment variable that I couldn't remember the exact name of, so I think it will be helpful to others as well. 

The document is structured as a bunch of tables, each with the flag/env var, a short description of what it does, and a link to further details where present. It also includes a brief glossary at the top and a "See Also" section at the bottom for the interested reader. 

## What changed

New file `CONFIGURATION.md` in `docs/` plus a link to it from `README.md`.

## Extra Notes
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