Document environment variable that disables printing of virtual environment name in prompt (#7648)
This PR adds a line to `docs/configuration/environment.md` that
documents `VIRTUAL_ENV_DISABLE_PROMPT`. If set to `1` when the virtual
environment is activated, then the virtual environment name will not be
prepended to a terminal prompt.
So far I've tested this in bash, but from the various activation
scripts, it looks like it is respected for a variety of shells.
Maintainers should please feel free to edit this PR directly. Thank you!