Docs: add migration guides (MVVM, in-house MVI, Orbit, MVIKotlin)

[Nek-12]

It would be really valuable to add a dedicated docs page (or a small section) focused on migrating to FlowMVI from common architectures and libraries.

Proposal

Add a new docs page under something like docs/docs/migrations/ (or docs/docs/guides/) that covers:

• MVVM → FlowMVI
• In-house MVI implementations → FlowMVI
• Orbit MVI → FlowMVI
• MVIKotlin → FlowMVI

What the page should include

• Concept mapping: ViewModel/StateFlow/LiveData → Store/State/Intents
• Typical project structure and where logic should move
• Step-by-step incremental adoption strategy (start with one feature/module)
• Common pitfalls & gotchas (side effects, threading, state restoration, testing)
• Examples/snippets for each source approach
• Testing guidance and how to keep existing tests while migrating

Why

Migration is the biggest adoption hurdle. A practical guide reduces time-to-first-success and helps teams evaluate FlowMVI against existing solutions.

Nice-to-haves

• A small comparison table (features + mental model)
• Links to relevant existing docs pages (Quick Start, plugins, debugging, metrics, testing)

[Nek-12]

Implementation Steps

1. Create a new markdown section under the docs directory (one markdown file per framework)
2. Add the pages to the mkdocs.yml navigation structure so it shows up in the sidebar.
3. Write files covering the following mappings:
   • MVVM → FlowMVI: (ViewModel / StateFlow / LiveData → Store / State / Intents)
   • Orbit MVI → FlowMVI: (ContainerHost / reduce / intent → Store / updateState / intent)
   • MVIKotlin → FlowMVI: (Store / Executor / Reducer / Bootstrapper → Store / intent / updateState / init)
   • In-house MVI → FlowMVI: etc...
4. Highlight typical project structure changes (where side-effects move, threading, state restoration).

This should feel like a step by step guide/template, that the agents/humans can mechanically follow to migrate screens, with mappings between popular APIs (e.g. repeatOnSubscription -> whileSubscribed), config options etc.