Enhance learning material code examples validation and formatting

[TrialDragon]

Objective

Part of #265
Fixes #279

As it stands code snippets used in the learning materials are not formatted and are currently only checked / validated by code-validation which is a generally fragile, and at times frustrating, tool.

Solution

This PR removes the code-validation tool and supplants it with learning-code-examples which is a Cargo project without a src/ folder which stores the learning material code examples in the examples/ directory. From here we can run cargo check --examples, cargo clippy --examples, and most importantly cargo fmt --check to validate and format the learning material code examples. Tho for ease of use it also supplies a validate_examples.sh script which runs all three of those commands on the examples.

The examples within have // ANCHOR: name, // ANCHOR_END: name and // HIDE to create the code snippets used in the pages.

The file_code_block shortcode also got refactored to accommodate this and is now the primary way to use code block examples in the learning materials.

The Quick Start Guide has been converted to use this tool as well.

Note

See the learning-code-examples/README.md for full usage details.

Note

This tool may also supplant write-rust-doc-lines, but I am unsure if write-rust-doc-lines impacts only the learning material code blocks, or if we also use it in the migration guides and other places in the site's content.

Testing

Run zola serve to see the results of the snippets being pulled in.
Run ./validate_examples.sh to see the code examples be confirmed as running. (This actually helped spot that the plugin-development generic plugin example was non-functional; now it should work).

[alice-i-cecile]

Delightful! Really pleased to see this working, and I think this is a reasonable approach.

[BD103]

Yay, I'm so happy this is finally happening! Thank you for your hard work :D

(I have a few nits, but nothing blocking.)

[BD103]

🥳