Add Migrating to Mill doc page

[lihaoyi]

This is a manual playbook for running such a migration, that will likely be useful even in the presence of semi-automated scaffolding generators

Resolves #4036

[lefou]

Looks good. I spotted some nits and added suggestions.

[jodersky]

Looks good! It might make sense to also mention using separate build files for large projects.

[jodersky]

def `ivyDeps` => `def ivyDeps`

[gamlerhart]

One challange I sometimes had to port builds:
The existing build tasks rely on stamping all over the file system to collaborate between parts. Like Maven puts something in one place, then a webpack step picks up from these pieces and places its stuff yet in another place. And so on and so forth. Especially if tools (webpack, bazel, etc) then rely on 'implicit' paths to find configuration etc.

This are portable, but sometimes a bit a head ache. You can give up and do dirty writes outside the Task.dest but then get the more fragile builds. Or be somewhat creative on how to wire the tasks so these stuff works better in isolation.

Anyway, not sure how to formulate this at this point. Probably would need a public example of this tension.