docs: add underscore parameter documentation

[crStiv]

Description

Problem*

Resolves #6917

Summary*

Added documentation for the underscore _ parameter feature in functions. This documents an existing feature where parameters can be marked with an underscore when they are not used in the function body, which is particularly useful for implementing traits or interfaces where not all parameters are needed.

Additional Context

The documentation was added to the Functions concept guide, specifically in the parameters section. The addition includes:

• Clear explanation of the underscore parameter feature
• Practical example showing its usage
• Follows the existing documentation style and formatting

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

Thank you for your contribution to the Noir language.

Please do not force push to this branch after the Noir team have started review of this PR. Doing so will only delay us merging your PR as we will need to start the review process from scratch.

Thanks for your understanding.

[TomAFrench]

Thanks for this PR. A couple of notes

[TomAFrench]

This function isn't valid noir as it's returning a value while the return type of the function is ().

[TomAFrench]

Noir actually ignores any parameter with a leading underscore so _x would also be accepted. This is preferred in general as it self-documents what the unused parameter is.

[crStiv]

like this? or did I not understand you right?

@TomAFrench