Documentation mentioning the fact that the logger outputs on STDERR would be helpful

[theredled]

As a new contributor, I spend some time searching for logs until I read mastodon/mastodon#23865

Mastodon is a 12-factor application and as such, logs to stdout.

Documentation could mention in "Troubleshooting / I'm not seeing much in my logs. How do I enable additional logging/debugging information?" section the fact that the logger log on STDERR and not #{Rails.root}/log/#{Rails.env}.log.
Would be helpful for new contributors.

https://docs.joinmastodon.org/admin/troubleshooting/#im-not-seeing-much-in-my-logs-how-do-i-enable-additional-loggingdebugging-information

[theredled]

#1604