[Cedar Doc Issue] Doc updates for 4.0.0

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

[x] Replace uses of "human", "human-readable" with "Cedar" https://github.com/cedar-policy/cedar/pull/1114
[x] Reserve __cedar identifier for internal use rfc#52
[x] Remove unspecified entities rfc#55
[x] Disallow shadowing definitions in the empty namespace rfc#70
[x] Allow EntityOrCommon field in JSON schema https://github.com/cedar-policy/cedar/pull/1060
[x] Disallow Bool, Boolean, Entity, Extension, Long, Record, Set, and String as common type names in schemas https://github.com/cedar-policy/cedar/pull/1150

cedar-policy / cedar-docs