docs: Add v1.38.0 to v2.0.0 migration guide

[spydon]

Description

Adds a new "Migration Guides" page to the docs describing the breaking changes users need to be
aware of when upgrading between major versions of Flame, along with the steps required to migrate.

This first version covers migrating from v1.38.0 to v2.0.0. To start with, it documents the one
breaking change that has landed on main since the v1.38.0 tag:

• GameWidget.controlled was renamed to GameWidget.managed (feat!: Rename GameWidget.controlled to GameWidget.managed #3961). The page explains the change
  and shows a before/after code snippet.

The page is registered in the Flame section's toctree (right after "Getting Started") so it is
discoverable in the rendered docs. As more breaking PRs land before the v2.0.0 release, new
subsections can be appended under the same version heading.

Changed files:

• doc/flame/migration.md (new page)
• doc/flame/flame.md (added the page to the visible list and hidden toctree)

Checklist

• I have followed the Contributor Guide when preparing my PR.
• [-] I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [-] I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

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