[Cedar Doc Issue] Doc updates for 4.0.0

[khieta]

Creating this issue to track the progress of doc updates for the newly released v4.0.0.

• Replace uses of "human", "human-readable" with "Cedar" Apply the Cedar schema terminology cedar#1114
• Reserve __cedar identifier for internal use rfc#52
• Remove unspecified entities rfc#55
• Disallow shadowing definitions in the empty namespace rfc#70
• Allow EntityOrCommon field in JSON schema Resolve types at fragment-combine time cedar#1060
• Disallow Bool, Boolean, Entity, Extension, Long, Record, Set, and String as common type names in schemas Implement #1139 cedar#1150