Add 3D Objects Counter+ update site

[Jay2owe]

Summary:

• Add the 3D Objects Counter+ update site to sites.yml.
• Verified the update site is live at https://sites.imagej.net/3DObjectsCounterPlus/ and db.xml.gz contains plugins/3D_Objects_Counter_Plus-0.1.0.jar.

Validation:

• python -m pip install -r requirements.txt
• python -m yamllint -d relaxed -d "{rules: {key-duplicates: {}}}" sites.yml
• duplicate id check: No duplicate IDs.
• alphabetical order check: Update sites are ordered correctly.
• $env:PYTHONUTF8="1"; python generate-legacy-pages.py
• python -m cram --shell "C:\Program Files\Git\bin\bash.exe" tests: 1 test passed, 0 failed.
• git diff --check