issue #36654-move environments doc to correct dir and update links

What does this MR do?

Issue #36654

It moves the content of doc/ci/environments.md to doc/ci/environments/index.md and updates the links to it in the other docs pages. In doc/ci/environments.md a redirect is left like in the documentation guide.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

The moved doc page is linked with many other docs pages. I used RubyMine which updated all the relative paths, so they should be checked in the review app to make sure everything still works.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Closes #36654 (closed)