retreat.ruby.org.au within the ruby_au codebase

[quintrino]

At the moment, retreat.ruby.org.au is a seperate repo.

https://github.com/rubyaustralia/Ruby-Retreat-2024

And hosted using a different server,

ideally this would be part of the core ruby_au repo, potentially with CODEOWNERS / submodules access provided to retreat organisers so they can update it directly without being given unlimited commit access to ruby_au

Useful to be able to have the design of the old retreats still visible at
retreat.ruby.org.au/2024 etc so that the details are kept rather than lost upon announcement of a new retreat.