Be explicit on JRuby being a requirement to create a custom plugin

Looking at the documentation pages for creating plugins, the only mention about JRuby being a requirement is in the [contributing a patch](https://www.elastic.co/guide/en/logstash/current/contributing-patch-plugin.html#_putting_it_all_together) page.

Pages like [How to write a logstash input plugin](https://www.elastic.co/guide/en/logstash/current/_how_to_write_a_logstash_input_plugin.html) should also inform of this dependency.

Start with the “How to write a logstash input plugin” documentation page and compare it with the linked “contributing a patch” page, where the JRuby requirement is already mentioned. Update the plugin-creation documentation pages to state that JRuby is required, then verify that the relevant plugin guides clearly communicate this dependency.

Written by the indexing model from the issue text.