Prep for v4.3.0

• Runs build/ship.sh 4.3.0
• Adds 4.3 to _data/docs-versions.yml for the docs Versions page
• Manually bumps the docs version in _config.yml
• Manually bumps the URLs for social images in _config.yml
• Runs git mv site/docs/4.2 site/docs 4.3

Need to verify any other directory paths to see if we've missed anything else.