Docs snippet-ification

[TomAFrench]

After #4164, we can now include code snippets from across the monorepo into the Noir docs. We've got a good amount of code-blocks in the docs which were copied across and so can be broken #4172. We should push any code blocks out of the docs and into tests/examples for which correctness is enforced in CI.

Getting Started

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1. Migrate https://noir-lang.org/docs/dev/getting_started/hello_noir/project_breakdown to pull from a concrete example project
2. Migrate https://noir-lang.org/docs/dev/getting_started/tooling/testing to pull from tests in the noir_test_success directory

Concepts

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1. Migrate "Concepts" over to pull from concrete example programs where possible.
2. Types of some array functions in the docs are incorrect #4172
   bug documentation +2

stdlib

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1. Pull blackbox function signatures from stdlib definitions #4176
   documentation enhancement +2
2. Move example usage of blackbox functions into execution_success tests.
3. Pull in Default trait definition #4180
   +0
4. Document EmbeddedCurvePoint along with add/double methods #4206
   +0
   
5. Document BigInt struct #4205
   documentation +1
   

Modules, Packages and Crates

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1. Create concrete example projects for "Modules, Packages and Crates".

How To Guides

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1. Rework https://noir-lang.org/docs/dev/how_to/how-to-recursion to pull snippets from a concrete example
2. Create a concrete example for https://noir-lang.org/docs/dev/how_to/how-to-oracles
3. Create a concrete example for https://noir-lang.org/docs/dev/how_to/merkle-proof

[signorecello]

@jzaki this could be a good first issue in Noir docs if you happen to have the availability