All proposals moving to enhancements repository

[shaneutt]

📣 A heads up that our previous proposal process (proposals go directly in the relevant repository) has stopped and we're now going to centralize them in https://github.com/praxis-proxy/enhancements.

Also note that the process around how proposals graduate and progress is taking new form, including an experimental stage where (by default) proposals will emit implementations into our https://github.com/praxis-proxy/experimental staging ground.

For full details check out the new documentation here.