Migrate documentation from wiki if needed

[basil]

If the plugin's <url> starts with http://wiki.jenkins-ci.org/display/JENKINS/ then its documentation needs to be migrated from the old wiki. This is a bit of a laborious process and I am not sure it can be fully automated. The steps are, roughly:

• Change the URL to <url>https://github.com/jenkinsci/${project.artifactId}-plugin</url>, unless of course the artifact ID is different from the GitHub repository.
• Fetch the wiki page from https://github.com/jenkins-infra/plugins-wiki-docs/ and split it from a single page into a README.md page and a CHANGELOG.md page. These files notoriously contain non-ASCII characters, so best to also do some Unicode normalization here.
• If the wiki export contains images, copy them from there to docs/images/.
• If the plugin is now using Release Drafter, add a note to CHANGELOG.md saying that future change logs are available at the GitHub releases page rather than in CHANGELOG.md.