Also clarify in build.ps1, build-steps.ps1 and build-package.ps1 that the
'.\tools\Build\...' example is a repository-root command and that callers in
other directories must provide a valid path to the scripts
Remove the one-off CI3->CI4 migration tooling that is no longer needed:
tools/ci4-lang-migration/ and tools/ci4-upgrade-language-interpolation/.
Docker is kept at the repository root rather than under tools/, per
review feedback. The Windows build scripts remain in tools/build/.
Root-relative docker paths are restored in .gitignore, BUILD.md,
INSTALL.md, gulpfile.js, and the build-release workflow.
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Co-authored-by: objecttothis <17935339+objecttothis@users.noreply.github.com>
