Add a 3.0 to 3.1 section to the upgrade guide #146

Summary

Add a 3.0 to 3.1 section to the upgrade guide (v3.0/upgrade-guide.md). 3.1.0 has several breaking changes, and the guide only covers 2.x to 3.0.

Why

Anyone who builds on the internals (events, the app and context classes, the debug toolbar, the mailer interface, the Lang config) has to change code when moving to 3.1. The framework changelog lists each change, but a reader needs what to change and why, with before and after examples.

What the guide should cover

The breaking entries of the 3.1.0 section in the framework CHANGELOG.md, each with what to change:

• the new minimum PHP version
• Hook replaced by Event (hook(), on(), fire(), shared/config/hooks.php, Debugger::HOOKS)
• Quantum\Loader removed and Setup moved to Quantum\Config\Setup
• the app boot and runtime as pipelines of stages: boot() on the adapters, the removed AppAdapter and traits, the moved namespaces, BootPipeline replaced by Pipeline, the AppContext getters that moved to WebContext, and AppContext's new required $type
• the console boot: CliCommand::$requiresEnvironment, the mutable environment, ConsoleAppAdapter::createApplication() removed
• the debug toolbar out of the app flow: InitDebuggerStage removed, the View constructor, MessageAdapter ignoring a tab key, MailerInterface::getTransportLog()
• the Lang configuration (lang.default, lang.default_locale, Lang::isEnabled() removed)

Acceptance criteria

• every breaking entry of 3.1.0 has a section with before and after code, or a clear "no action needed"
• the section is linked from the framework changelog and from the docs introduction
• the examples are checked against the released 3.1.0 code

Notes

• Write it on the staging branch; it ships with the 3.1.0 release.
• Depends on the 3.1.0 breaking changes being final, so the last pass comes just before the release.