Add documentation for Void XBPS package manager

[JuniorSuperTux]

Added installation documentation for Void XBPS package manager

Documentation change checklist

• My documentation changes follow the docs style guide and any code examples follow the coding style guide.
• I tested that my change works before raising the PR (via running doxygen, and testing examples).
• I have not moved any existing pages or changed any existing URLs without strong justification as to why.
• I have not generated content using AI or a desktop utility such as grammarly.

[CLAassistant]

All committers have signed the CLA.

[github-actions]

Thank you for your contribution, but PRs must be raised against the dev branch. Please log your pull request against the dev branch not master. You can also retarget this pull request, then reopen it.