Document local macOS/Arm runnable build workflow

[moreaki]

There is no concise technical note for creating a runnable local macOS/Arm VirtualBox.app while iterating on host-side code.

The needed information is currently scattered across configure output, kBuild overrides, staged app layout and Darwin signing logic. In practice, local developers need to know at least:

• the configure shape for a non-hardened OSE developer build;
• how to provide Xcode SDK/tool overrides when Oracle-style bundled tool trees are not present;
• how to build and refresh a selected host-side target in a staged app bundle;
• the signing order for changed Mach-O images, nested VM bundles and the outer app bundle;
• which VM entitlements are suitable for local developer testing and which release entitlements should not be added to ad-hoc/local Developer ID builds.

A short doc/technical page would make this workflow reproducible and reduce time lost to local signing and entitlement mistakes. The note should explicitly describe this as a local developer workflow, not a distribution packaging recipe.

[moreaki]

Re-audit update: the documentation PR #753 has been withdrawn because the instructions are not independently runnable on current main. The workflow currently depends on the Homebrew Qt configure fix in #747, shell-safe generated environment values in #752, and the LLVM 15 VBoxDxMt build fix in #748. This documentation request remains valid, but the final instructions should be recreated and tested end-to-end only after those prerequisites are available.