warn if denv init is run within a denv

[tomeichlersmith]

Is your feature request related to a problem? Please describe.
Sometimes folks run denv init within a subdirectory of a denv. This is supported but may lead to a lot of confusion, so adding a warning and confirmation to the user would be helpful to make sure they know what they are doing.

Describe the solution you'd like
If denv is able to deduce a workspace during denv init, there should be a warning and a prompt to the user even if the deduced workspace is in a parent directory and would not be overwritten.

Describe alternatives you've considered
Documentation additions which I think are important anyways.

Additional context
LDMX-Software/ldmx-sw#1472