docs: added an OpenBSD page

[Jaskowicz1]

This PR adds a page for building D++ on OpenBSD. The guide has been followed on a fresh install of OpenBSD and a fresh install of D++ (latest master) and works.

Documentation change checklist

• My documentation changes follow the same style as the rest of the documentation and any 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.

[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.