Scrape doc examples (#3334)

## Motivation and Context
Extracted from #3308, this adds doc example scraping. This won't make
much of a difference yet but it will help if we're able to move example
repos directly under the SDKs themselves when appropriate.

## Description
https://doc.rust-lang.org/rustdoc/scraped-examples.html

## Testing
verified that examples appear in docs when expected

## Checklist
no changelog

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