docs: move the root-level build docs under docs/build/

build-release.md, build-desktop-packages.md and build-windows.md sat at
the docs/ root while docker.md and build-builder-image.md were already in
docs/build/, so "where do build docs live" had two answers. They now have
one.

Referrers updated: README.md, docs-site/SUMMARY.md, and the ../ links
inside the moved files themselves, which each gained a level of depth —
Dockerfile, Dockerfile.arch, packaging/arch/PKGBUILD, CHANGELOG.md,
README.md, src-tauri/src/lib.rs and src/lib/services/webviewAudio.ts.
Every one of those was caught by check-doc-links.sh rather than by
reading, which is the point of having it.

Two referrers are left for their owners: CLAUDE.md line 173 and the
comment at scripts/build-windows-cross.sh line 11.

README.md

@@ -42,7 +42,7 @@ For the full set of build, test, and Android helper scripts, see
 |-------|----------|
 | Architecture overview & subsystem docs | [docs/architecture/](docs/architecture/) |
 | Requirements, traceability & technical debt | [docs/requirements.md](docs/requirements.md) |
-| Build & release process | [docs/build-release.md](docs/build-release.md) |
+| Build & release process | [docs/build/build-release.md](docs/build/build-release.md) |
 | Docker builds | [docs/build/docker.md](docs/build/docker.md) |
 | Traceability tooling & CI | [docs/traceability.md](docs/traceability.md), [docs/traceability-ci.md](docs/traceability-ci.md) |
 | Release checklist | [docs/release-checklist.md](docs/release-checklist.md) |