Update the rest of the QEMU chapter for app-template

[webbgamers]

The cargo-generate command in this chapter switched to knurling's app-template (and #418 updates the surrounding project-creation prose to match), but everything downstream of project creation still describes cortex-m-quickstart and the pre-defmt hprintln! workflow. Following the chapter as written does not currently produce a running program.

Program Overview: show src/bin/hello.rs rather than a src/main.rs that the template does not have, and explain use app as _; instead of panic_halt.

Cross compiling: the template defaults to thumbv6m-none-eabi, so add the missing step of switching TODO(3) to thumbv7m-none-eabi. The listing shown before was quickstart's, which had the Cortex-M3 target already selected.

Inspecting: the template builds no binary named after the project, so cargo readobj --bin app fails; use --bin hello. Refresh the captured output and describe every section the output lists -- the refresher previously covered five of them and lumped the rest together as "metadata" -- along with flip-link's inverted RAM layout, which is what readers will actually see.

The disassembly excerpt was a truncated dump of __stext alone. Show __stext and main together via --disassemble-symbols, which needs no truncation and covers the whole boot path: pre-init hook, zeroing .bss, copying .data, then the call into the entry point. -C demangles the call targets.

Running: plain qemu-system-arm cannot decode defmt, as the chapter itself says two paragraphs earlier, so it can't be the runner and doesn't print "Hello, world!" -- it prints four bytes of binary frame. Set qemu-run as the runner instead, under the cfg() table the template actually uses. Install qemu-run from crates.io rather than cloning the defmt repo; the clone snippet also used an SSH URL and a ../qemu-rs/ path that does not exist. Fix --example hello to --bin hello.

Debugging: pass the GDB flags through qemu-run so the log output is still decoded, correct the examples/hello paths, and replace the hprintln! / debug::exit listing with the program the template ships. cortex-m-rt writes Reset in assembly now, so GDB stops at a bare address with a misattributed symbol rather than at cortex-m-rt-0.6.1/src/lib.rs:473.

install/tooling: add flip-link, which the template sets as its linker and without which the build fails, and qemu-run. Neither was mentioned anywhere.

All commands and output were re-run against a fresh app-template project.

Semihosting, Panicking and Exceptions still print with hprintln!, and qemu-run swallows that plain text silently rather than erroring, so the chapter now tells readers to switch the runner back to plain qemu-system-arm before those chapters. Migrating them to defmt would remove the caveat and is probably worth doing.

The entry link also now points at cortex-m-rt's re-export rather than the cortex-m-rt-macros page, which carries no documentation of its own.