Add Macro Builder update site

[Jay2owe]

Summary

• Add the Macro Builder Fiji/ImageJ update site to sites.yml.
• Link the listing to the public Macro Builder repository and add Jamie Malcolm as maintainer.

Validation

• python -m yamllint -d relaxed -d "{rules: {key-duplicates: {}}}" sites.yml passed.
• Duplicate ID check passed: no duplicate IDs.
• Git Bash .github/build.sh passed YAML lint, duplicate ID, alphabetical order, and legacy page generation with PYTHONUTF8=1 set.
• python -m cram --shell "C:\Program Files\Git\bin\bash.exe" tests passed: 1 test, 0 failures.
• git diff --check passed.

Note: On Windows, the build script's plain cram tests invocation looks for sh; the Cram test was run with Git Bash explicitly, as recommended for Windows.