Add CircleCI migration document

What does this MR do?

This MR adds a guideline for migrating from CircleCI to GitLab. The information was written from the perspective of someone using CircleCI terminology (Headline is Workflows as opposed to Stages). Also included are side by side code comparisons between yaml. The document has the following outline:

• Intro
• config.yaml vs gitlab-ci.yml
  • Job
  • Docker Image Definition
  • Workflows
    • Parallel and Sequential Setup
    • Scheduled Runs
    • Manual Run
  • Filter Job by Branch
  • Caching
• Contexts and Variables
• Orbs
• Build Environments
  • Self-managed runner
  • GitLab.com Shared Runners
  • Machine and Specific Build Environments

Issue: #211441

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