docs: add migration guide from 0.12 to bonsai

[Walther]

This adds a rudimentary migration guide from 0.12 to Bonsai.

This is a bit of a minimum viable tutorial for pre-release purposes only. We should replace this with better documentation and a blog post later on.

Squash before merging to clean up the history.

[vvagaytsev]

LGTM! 👍

Very good initial version! Let's apply further improvements on top of it.
I've let a couple of comments, feel free to address/discuss :)

[Orzelius]

I think the following is worth mentioning:

• Where is the reference documentation for actions
• Modules can't depend on actions
• Modules can't reference actions
• Actions can reference and depend on modules, by referencing the actions that are generated from modules
• Converting your project to the actions config can be performed gradually by starting from the end of the dependency tree

I think adding this resolution flow might make it more understandable:

1. modules are resolved
2. modules are converted to actions
3. actions are resolved

[vvagaytsev]

Should we also include #3512 in the breaking changes list?

[vvagaytsev]

Great job, thanks! 🚀