Graylog2 "How-to guide" can be updated to use RubyGems

[nosas]

Describe the bug

When configuring Fluentd, the documentation states that the GELF plugin is not available on RubyGems:

Then, install the out_gelf plugin to send data to Graylog. Currently, the GELF plugin is not available on RubyGems, so we need to download the plugin file and place it in /etc/td-agent/plugin:

$ wget https://raw.githubusercontent.com/emsearcy/fluent-plugin-gelf/master/lib/fluent/plugin/out_gelf.rb
$ sudo mv out_gelf.rb /etc/td-agent/plugin

Link to the problematic documentation

https://docs.fluentd.org/how-to-guides/graylog2#fluentd

Expected explanation

The fluent-plugin-gelf is available on RubyGems under many names. Most notably as fluent-plugin-gelf and fluent-plugin-gelf-hs. The latter is a fork of emsearcy's gelf plugin as mentioned in Fluentd's doc above.

Additional context

No response