Documentation update

[Logofile]

This change includes the following:

• Updates links to /programming-guides/proto2.md throughout topics
• Adds content to editions/features.md about group-like field behaviors.
• Adds content to editions/features.md about the features.(pb.cpp).string_type feature.
• Adds a new topic, reference/go/size.md
• Updates guidance on releases in support/version-support.md