Use gh pages for update site

[adangel]

Currently we use bintray for hosting the update site, but this will soon be gone (May, 1st 2021): https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter/

The simplest solution would be, to create a new repository "pmd-eclipse-plugin-p2-site" which is used as a github pages site. The update site URL would then be https://pmd.github.io/pmd-eclipse-plugin-p2-site/

See https://pmd.github.io/pmd-eclipse-plugin-p2-site/
(Repo: https://github.com/pmd/pmd-eclipse-plugin-p2-site)