Add docs for the human-readable schema format

[shaobo-he-aws]

Issue #, if available:

86

Description of changes: This PR also restructures the schema docs.

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[shaobo-he-aws]

I don't know if we want to present the "human-readable" and "json" schemas as two alternate syntaxes. It seems like it would be more consistent with the presentation of policies to focus on the new schema syntax (calling it "schema syntax" rather than "human-readable schema syntax") and then have a separate page about how to encode schemas in JSON.

I thought about it. IIRC, we decided to use the name "Cedar schema syntax" after a transitional period during which we adopt the term "human-readable schema syntax".

[mwhicks1]

I thought about it. IIRC, we decided to use the name "Cedar schema syntax" after a transitional period during which we adopt the term "human-readable schema syntax".

I agree with just calling it "Cedar schema syntax" eventually. In the meantime: I have seen others use the term "natural syntax". Originally, I was calling it "custom syntax." I find both of these better than "human-readable" but I don't mind too much either way.

[cdisselkoen]

The Cedar Rust APIs use "natural" (eg, Schema::from_file_natural()). The Cedar CLI uses "human" because it already uses "human"/"json" to distinguish the two policy formats.

In the docs, I agree with @khieta that we should be consistent with the presentation of policies, where we just present the natural syntax as the obvious one, and have a separate page where we discuss the JSON format but don't give it equal billing.