docs(python): Update Flask quick start guide

[inventarSarah]

DESCRIBE YOUR PR

Updated the Starlette quick start guide to follow our quick start guide template.

• split layout
• follows general structure of quick start guide
• adds "metrics" to onboarding options
• adds more Verify examples
• uses SdkOption for a clean options list
• adds a new tab for the stream mode specific tracing verify code snippet (note that this is an include and will be used in all Python quick start guides that use it)

Closes: #19260

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