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.

docs-site/SUMMARY.md

@@ -71,10 +71,10 @@
 
 # Build & Release
 
-- [Build & Release](build-release.md)
+- [Build & Release](build/build-release.md)
 - [Release Checklist](release-checklist.md)
-- [Desktop Packaging](build-desktop-packages.md)
-- [Windows Build](build-windows.md)
+- [Desktop Packaging](build/build-desktop-packages.md)
+- [Windows Build](build/build-windows.md)
 - [Defect Windows](defect-windows.md)
 - [Docker](build/docker.md)
 - [Builder Image](build/build-builder-image.md)