Adds GitLab's public PlantUML server to docs for PlantUML integration

4 jobs for !18580 with jarv/public-plantuml-for-self-managed in 2 minutes and 41 seconds (queued for 157 minutes and 36 seconds)
detached
Status Job ID Name Coverage
  Test
failed #321449687
gitlab-org
danger-review

00:02:41

failed #321449685
gitlab-org
docs lint

00:01:04

 
  Review
skipped #321449689
gitlab-org allowed to fail manual
review-docs-cleanup
skipped #321449688
gitlab-org allowed to fail manual
review-docs-deploy
 
Name Stage Failure
failed
danger-review Test
```
bin/changelog --ee -m 18580 "Adds GitLab's public PlantUML server to docs for PlantUML integration"
```

Note: Merge requests with ~backstage, ~ci-build, ~meta do not trigger this check.
Running after script...
$ date
Tue Oct 15 15:13:14 UTC 2019
ERROR: Job failed: exit code 1
failed
docs lint Test
=> Checking for executable permissions...
=> Checking for new README.md files...
✔ Linting passed
$ markdownlint --config .markdownlint.json 'doc/**/*.md'
doc/administration/integration/plantuml.md: 21: MD034/no-bare-urls Bare URL used [Context: "https://plantuml.gitlab-static..."]
Running after script...
$ date
Tue Oct 15 15:11:38 UTC 2019
ERROR: Job failed: exit code 1