Duplication of importing environments documentation

[interurban]

Noted by @IaroslavTitov :
"I noticed that we have exact same blob about importing environments in "Working with Environments" page. I think we should remove it out of there, since we have this separate page, or at least also update it there"

The opening paragraphs on these two docs pages are duplicative:
https://www.pulumi.com/docs/esc/environments/working-with-environments/#importing-other-environments
and
https://www.pulumi.com/docs/esc/environments/imports/

The working with section should explain imports at a high level and reference the detailed sub-page