👷 CI: Add GitLab CI for Antora

This commit adds a GitLab CI configuration to automatically test and
build previews for new Merge Requests submitted against this repository.
It provides an easier way to validate changes before merging them to the
`main` branch.

Signed-off-by: Justin W. Flory (he/him) <jwf@redhat.com>

.gitlab-ci.yml

@@ -0,0 +1,28 @@
+build:
+  stage: build
+  image:
+    name: antora/antora:latest
+  script:
+  - antora --log-failure-level fatal site.yml
+  artifacts:
+    paths:
+    - public/
+    expire_in: 1 week
+    expose_as: 'docs preview'
+  rules:
+  - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+  environment:
+    name: preview MR_${CI_MERGE_REQUEST_IID}
+    url: https://${CI_SERVER_HOST}/${CI_PROJECT_PATH}/-/jobs/${CI_JOB_ID}/artifacts/file/public/index.html
+    auto_stop_in: 1 week
+pages:
+  stage: build
+  image:
+    name: antora/antora:latest
+  script:
+  - antora site.yml
+  artifacts:
+    paths:
+    - public
+  rules:
+  - if: $CI_COMMIT_BRANCH == "main" && $CI_PROJECT_ROOT_NAMESPACE == "fedora"