Replace "Edit on GitHub" workflow with contribution page

[alexander-nitsche]

Most of the VHS documentation is generated from the VHS source code (after #1771 will have been merged). Only a minor set of pages is maintained manually. Therefore, the "Edit on GitHub" workflow is counterproductive because it applies PRs to the reST files that will be overwritten during the next generation run. Remove the "Edit on GitHub" workflow and instead add a contribution page that explains all the manual steps.