Add collaboration workflow to getting started guide

[erak]

Description

One of the learnings from last weeks onboardings is that some users did not know "what to do" with a newly created Radicle project. Others were either interested in passively mirroring their existing projects or actively experimenting with code collab.

Proper mirroring requires changes to moving parts (identity, delegation etc.), which should be done when it's more clear how they will work in the future. Documenting the current collaboration flow should be a more or less independent task, that takes the opportunity of the website's restructuring to make it available on https://radicle.xyz/get-started.html.

Specification

The update of the getting started guide could be split up into two steps:

1. Add the non-Upstream parts from https://docs.radicle.xyz/using-radicle/create-submit-patch and https://docs.radicle.xyz/using-radicle/track-review-merge
2. Write-up workflow based on scripts/contributor-workflow-setup.sh and mark it as experimental