[Hacktoberfest] Migrate docs to jenkinsci repo

[Dan-Heath]

This PR migrates the Custom Build Properties documentation from the plugins-wiki-docs repo to the custom-build-properties repo as part of the Hacktoberfest docs-to-code project. Since the documentation for this plugin was already in the README, all I did was update the pom.xml file with the new URL for the docs.

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[Dan-Heath]

Hello @shasait , I created this PR as part of the Hacktoberfest docs-to-code project. All this plugin required was an update to the pom.xml file. I think the PR is ready for review. Please take a look and let me know if you have any questions or if there are any changes you'd like me to make. Thank you!

[shasait]

Thank you very much for your contribution!