[armanist]

Closes #146. Documents the breaking changes of 3.1.0, as listed in the framework changelog (quantum-php/framework#609).

What changed

v3.0/upgrade-guide.md becomes a two-part guide: "Upgrading from 3.0 to 3.1", new and first, and "Upgrading from 2.x to 3.0", unchanged except that its headings go down one level so they nest under the part title. Each 3.1 section says what to change, with before and after code where code changes, or that no action is needed, and links to the package page that explains the topic:

1. Platform and dependencies: PHP 8.1, Symfony 6.4, psr/log 2 or 3, the Logger message type, the ResponseHeaders and DatabaseHandler return types
2. OpenAPI documentation is written as attributes, and doctrine/annotations is no longer required
3. Deprecations are logged, not thrown
4. Hook is replaced by Event
5. Quantum\Loader is removed, and Setup moved to Quantum\Config\Setup
6. The app boots and runs through pipelines of stages
7. The console boots in explicit steps
8. The debug toolbar reads its data when it renders
9. The Lang configuration selects an adapter
10. A checklist, including a grep for the replaced names

Every example was checked against the framework code on master and against 3.0.3.

Changed paths

• v3.0/upgrade-guide.md
• SUMMARY.md: the entry is renamed from "Upgrade Guide (2.x to 3.0)" to "Upgrade Guide"

Notes for reviewers

• The ticket asks for a last pass just before the 3.1.0 release, because the examples must match the released code.
• Once the first documentation pull request is merged, the @OA note in cli/code-generation.md can shrink to a link to section 2 of the guide.