Update README for v3

[hossam-nasr]

Update the README for the v3.x branch so that it's accurate to the new prog model. Changes made:

• Update build status buttons to reference the v2.x and v3.x branches instead of dev and main
• Include references to docs about the new prog model and the new upgrade guide
• Update existing links to link to the v4 model tab/pivot when applicable
• Update code samples to the v4 model samples
• Update broken/deprecated/moved links