Minor updates to snippets for doc restructuring

[vinishavathwani]

Made the following changes:

• Updated comments for clarity
• Replaced random example values with placeholders
• Added snippet SignInJsonRequest to indicate the json object sent by the server to the client app during sign in
• Update credentials dependencies to "1.6.0-beta03" (required to use conditional create)
• Updated the parameter for Conditional Create to be isConditional instead of isConditionalCreateRequest

[snippet-bot]

Here is the summary of changes.

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• identity/credentialmanager/src/main/jsonSnippets.json:127, tag android_identity_create_sign_in_request_json

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