Add CI_OPEN_MERGE_REQUESTS environment variable

What does this MR do?

Add CI_OPEN_MERGE_REQUESTS environment variable which only exists if pipeline has open merge request.

Currently it is not possible to run pipeline for branch only when merge request does not exist. This enables that and enables ability to remove duplicate pipelines as discussed at #201845 (closed) like follows:

workflow:
  rules:
    - if: $CI_MERGE_REQUEST_IID
    - if: $CI_OPEN_MERGE_REQUESTS
      when: never
    - if: $CI_COMMIT_BRANCH

Fix #201845 (closed).

Afterwards a number of documentation tweaks can be made (e.g. update details added as part of !36009 (merged)).

• Create separate issue for this

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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