refactor: move build scripts to build/ directory

Declutters the repo root by consolidating build-common.sh,
build-dist.sh, build-dist-windows.sh, build-dist.ps1, and installer.nsi
into build/. Updates all path references in CI workflows, NSIS installer,
and documentation.

.gitignore

@@ -4,7 +4,16 @@ __pycache__/
 *$py.class
 *.so
 .Python
-build/
+# Build output artifacts (LedGrab/, *.zip, *.exe, *.tar.gz, cached downloads)
+build/LedGrab/
+build/*.zip
+build/*.exe
+build/*.tar.gz
+build/*.msi
+build/python-embed-*.zip
+build/pip-wheels/
+build/win-wheels/
+build/tk-extract/
 develop-eggs/
 dist/
 downloads/