docs: create Gentoo installation tutorial

[folospior]

I'm one of the Gentoo maintainers for D++, so I decided to make a page on how to install it on Gentoo.

The tutorial explains how to enable GURU, how to install D++ using portage, how to enable voice and coro support, and finally how to build a bot using G++, as I noticed is the case with all other installation guides.

Documentation change checklist

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

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