CI: add mdbook test to validate guide in PRs

[benhillis]

This change ensures that code samples in the rust code samples compile and fixes any current issues preventing mdbook test from currently passing.

[Copilot]

Pull request overview

This PR adds mdbook test to the CI build process to validate that Rust code examples in the guide compile correctly. It also marks several flowey-related code snippets with rust,ignore to exclude them from compilation testing, and removes a redundant configuration setting from book.toml.

Changes:

• Added mdbook test command to build_guide.rs to validate code examples during guide builds
• Marked incomplete code snippets in flowey documentation files with rust,ignore attribute
• Removed redundant multilingual = false setting from book.toml (it's the default value)

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
flowey/flowey_lib_hvlite/src/build_guide.rs	Added mdbook test command and refactored environment variable setup
Guide/src/dev_guide/dev_tools/flowey/variables.md	Marked code snippets with rust,ignore
Guide/src/dev_guide/dev_tools/flowey/steps.md	Marked code snippets with rust,ignore
Guide/src/dev_guide/dev_tools/flowey/pipelines.md	Marked code snippets with rust,ignore
Guide/src/dev_guide/dev_tools/flowey/nodes.md	Marked code snippets with rust,ignore
Guide/src/dev_guide/dev_tools/flowey/artifacts.md	Marked code snippets with rust,ignore
Guide/book.toml	Removed redundant default configuration value

[github-actions]

At least one Petri test failed.