Docs: move environments doc into correct dir

Problem to solve

We have an inconsistency in the CI environments documentation structure.

The index is placed under doc/ci/ (https://docs.gitlab.com/ee/ci/environments.html), while the protected environments doc is correctly placed into doc/ci/environments/ https://docs.gitlab.com/ee/ci/environments/protected_environments.html

Proposal

Move /ci/environments.md into doc/ci/environments/index.md

Who can address the issue

Anyone. It's ~"Good for 1st time contributors"