[guide] Remove ambiguity lib vs. executable

[syl20bnr]

Checklist

• Confirmed that run-checks all script has been executed.
• Made sure the book is up to date with changes in this PR.

Changes

This PR transforms the guide example into a regular Rust executable so that people following the Basic Workflow in the Burn book are not surprised that the guide source does not correspond to their project.

In order to maintain the possibility to use the guide source as a library like it is the case in the custom-renderer example, a lib.rs file is added with a comment at the top for users.

In order to maintain the possibility to execute the guide as a regular example the examples/guide.rs has been changed to simply call cargo. The downside is that a new executable has to be compiled.

Testing

cargo run --release --bin guide
cargo run --example guide
cargo run --example custom-renderer

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.45%. Comparing base (6ae3926) to head (4da8c2f).
Report is 2 commits behind head on main.

❗ Current head 4da8c2f differs from pull request most recent head 7eb0f1a. Consider uploading reports for the commit 7eb0f1a to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1649   +/-   ##
=======================================
  Coverage   86.45%   86.45%           
=======================================
  Files         696      696           
  Lines       81236    81236           
=======================================
  Hits        70233    70233           
  Misses      11003    11003           

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[louisfd]

I have minor requests, otherwise looks good! Thanks for the change I think it will be much clearer

[antimora]

LGTM