docs/tutorial/ch2: add term definitions and section transitions

[zevorn]

Summary

• Add inline concept introductions for terminology at first occurrence across 10 ch2 articles (accel, SysBus, chardev, vmstate, icount, BAR, BQL, cmodel, bindgen, etc.)
• Add transition paragraphs between sections to improve reading flow and logical coherence
• 5 articles (cpu-model, softmmu, intr, gpgpu-kernel, rust-test) already had good flow and were left unchanged

Changed files

• qemu-tcg.md — 6 transitions + accel definition
• qemu-insn.md — 1 transition (decode → implement)
• qemu-multi-core.md — 4 transitions + BQL definition
• qemu-hw.md — 5 transitions + SysBus/chardev/vmstate definitions
• qemu-clock.md — 4 transitions + icount definition
• qemu-pcie.md — 3 transitions + BAR definition
• qemu-gpgpu.md — 5 transitions + cmodel definition
• qemu-rust-ffi.md — 4 transitions + bindgen definition
• qemu-rust-model.md — 5 transitions + BQL definition
• qemu-machine.md — 2 transitions

Test plan

• make format passes with no changes
• Verify all articles render correctly in MkDocs
• Review added content for technical accuracy