Reduce a few png files in size using pngcrush

Details:

assets/images/6.2-blog/[email protected]: reduced by 22503 bytes (43.39%)
assets/images/getting-started/cloud-services/vapor-logo-dark-background.png: reduced by 12738 bytes (44.00%)
assets/images/swift-redesign-blog/swift-redesign-blog.png: reduced by 13724 bytes (1.95%)
assets/images/getting-started-with-vscode-swift/installation.png: reduced by 392912 bytes (38.76%)
assets/images/6.2-blog/[email protected]: reduced by 449932 bytes (47.59%)

Author: tkremenek