[Hacktoberfest] Migrate docs to jenkinsci repo

[Dan-Heath]

This PR migrates the Mock Load Builder plugin docs from the wiki to GitHub as part of the Hacktoberfest docs-to-code project. All that was required for this plugin was to update the pom.xml file with the new URL for the docs.

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[Dan-Heath]

Hi @Vlatombe and @jglick , I updated the pom.xml file with the new URL for the docs, and you already had a README. I think this is ready for review. Please let me know if there's anything else I can do. Thank you!

[jglick]

Seems https://github.com/jenkins-infra/plugins-wiki-docs/blob/master/mock-load-builder/README.md was never anything interesting. Thanks!