Release Process Cleanup: Cloud Controller Manager

[sudomateo]

Let's clean up the release process across our repositories so that the release process is documented in a consistent location.

• Create a top-level CONTRIBUTING.adoc file. If there's an existing CONTRIBUTING.md file, rename it to CONTRIBUTING.adoc.
• Document the release process in a new or existing section within CONTRIBUTING.adoc.
• Delete the .github/ISSUE_TEMPLATE/release_checklist.md file and any other issue templates that have to do with releasing except for release.yaml. Be sure to synthesize the instructions from these deleted files into CONTRIBUTING.adoc.