Add union and enum doc and refine logic

[tadelesh]

fix: #379
resolve: #385

[azure-sdk]

All changed packages have been documented.

[azure-sdk]

You can try these changes at https://cadlplayground.z22.web.core.windows.net/typespec-azure/prs/389/

Check the website changes at https://tspwebsitepr.z22.web.core.windows.net/typespec-azure/prs/389/

[tadelesh]

@iscai-msft I saw you rollbacked my change for union as enum with hierarchy. I'm wondering do we have the agreement of erasing the hierarchy types and flattened into one layer of enum? It will break JS RLC, since the type has been changed. For the following example, I don't think we could omit BaseEnum and just generate ExtendedEnum as enum. I prefer to generate ExtendedEnum as union and BaseEnum as enum. Also, do we have the agreement of not generating anonymous union as enum? I don't like the inconsistency.

union BaseEnum {
  string,
  EnumValue1: "value1",
}

union ExtendedEnum {
  BaseEnum,
  EnumValue2: "value2",
}

[iscai-msft]

@tadelesh this was after talking with @lmazuel , but I'm going to start a discussion in dpg sync about this

[iscai-msft]

here's the gist I wrote up https://gist.github.com/iscai-msft/4174b2e13811c13d91af86c1296e9ed1

[tadelesh]

/azp run

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[tadelesh]

/azp run

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