WikiFormatting: add deprecation notice for `wiki_to_html`, `wiki_to_oneliner` and `wiki_to_outline`. Use the `format_to...` methods instead.
There's no equivalent yet for `wiki_to_outline`, so this one should still be used for now. For the next major version, where the most likely replacement API for this will be `format_to(env, 'outline', ...)`.
