Release generation via gitlab-ci.yml documentation

What does this MR do?

Adds docs for the release pipeline stage defined in issue #199250 (closed)

release_job:
  stage: 'release'
  only: 'tags'
  script:
    - echo 'running release_job'
  release:
     name: 'Release $CI_COMMIT_SHA'
     description: 'Created using the release-cli $EXTRA_DESCRIPTION'
     tag_name: 'release-$CI_COMMIT_SHA'
     ref: '$CI_COMMIT_SHA'
     milestones: 
       - 'm1'
       - 'm2'
       - 'm3'
     released_at: '2020-07-15T08:00:00Z'

Related issues

Part of #199250 (closed)

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• [-] If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.

/label documentation