Add docs about moving from plugin.path to Gem based plugins

With the deprecation of `path.plugins` in https://github.com/elastic/logstash/pull/6439, we need to provide an "upgrade path" for the users using `path.plugins` to Gem based plugin development.

Start by reading the deprecation change in pull request 6439 and identify how users currently use path.plugins. Document the upgrade path to Gem-based plugin development, including the old and recommended approaches. Done means the documentation explains how affected users can migrate.

Written by the indexing model from the issue text.