Fix Sandcastle local build

[jjspace]

Description

I noticed a small bug that was incorrectly building the Sandcastle standalone.html file into the wrong directory.

• Fix the build directory for sandcastle.html when run locally
• Add gitignore files for the Sandcastle rework Rebuilding Sandcastle: Technical Implementation #12566 to avoid extra files tagged from git when switching back and forth from those branches.

Issue number and link

No issue

Testing plan

• Delete the directories Build/Apps and Build/Sandcastle if they exist
• run npm run build-apps and check that Build/Apps/Sandcastle has all files including standalone.html
  • Specifically check that Build/Sandcastle does not exist and contain standalone.html (this happens on main)
• Run PROD=true npm run build-apps and make sure that Build/Sandcastle has all files including standalone.html

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @jjspace!

✅ We can confirm we have a CLA on file for you.

[ggetz]

Thanks @jjspace! Looks good to me, both locally and in CI.