Add docs page about environment variables

As we've built out more configuration by environment variables, we need to update the docs to explain what functionality is out there.
Examples:
`$CUE_FRAME_LOG_DIR `
`$CUESUBMIT_CONFIG_FILE`

No files or tests are named. Start by locating the existing configuration documentation and the implementations or references for CUE_FRAME_LOG_DIR and CUESUBMIT_CONFIG_FILE; done means adding a docs page that explains the available environment-variable functionality and includes these examples.

Written by the indexing model from the issue text.