Documentation for breaking release workflow

> Do we already have a section where we explain the general workflow for a breaking release? > This is not directly related to the schedule but I think it would be helpful to at least point to this workflow from here.
>
> Some points that are important in my opinion:
>
> * What we did last time: At some point there was a `v3_develop` branch in parallel to the `develop` branch. At some later point we decided to merge `v3_develop` into `develop`. This closed the door for any v2 releases and forced everybody to integrate new features directly with v3.
> * I think it would also be helpful to point to the milestone that we usually create in order to cluster breaking releases way before the breaking release is scheduled (see https://github.com/precice/precice/milestone/20).

_Originally posted by @BenjaminRodenberg in https://github.com/precice/precice.github.io/pull/475#discussion_r1872943969_