Move URL Rewriter 2.1 and lower from Gitlab to GitHub Forge

Move the 2.1 and lower versions of https://code.onehippo.org/cms-community/hippo-plugin-urlrewriter to GitHub Forge https://github.com/onehippo-forge/url-rewriter 

• do not move the master branch there: it was meant for Hippo 12
• make a documentation site https://onehippo-forge.github.io/url-rewriter/ with a reference to it at https://onehippo-forge.github.io/project-list.html