cpp11-migrate: Docs refresh

* Documented new command-line options.
* Moved usage to a new page.
  * Usage now split into general options and transform-related options.
* Main Migrator page now contains getting started and getting involved
  information.
  * Also included a JIRA issue collector button for logging bugs.



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@183417 91177308-0d34-0410-b5e6-96231b3b80d8