Document provider IDs

The page at https://maven.apache.org/scm/scm-url-format.html currently does not explain which values are accepted for scm_provider out of the box. Also the linked page https://maven.apache.org/scm/scms-overview.html does not contain the according SCM provider IDs. The native ID e.g. for the JGit provider (jgit) is nowhere documented. Also the mapping of providers via plugin parameter providerImplementations (supported in m-release-p and m-scm-p) should be explained there.