docs: create Gentoo installation tutorial

docpages/install/install-gentoo-portage.md

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+\page install-gentoo-portage Installing with Portage (Gentoo)
+
+D++ is available on [GURU](https://wiki.gentoo.org/wiki/Project:GURU). To install D++, you must first enable the GURU repository.
+To do so, execute the following commands (as root):
+
+```bash
+emerge --ask app-eselect/eselect-repository
+eselect repository enable guru
+emaint sync --repo guru
+```
+
+This enables the GURU repository, which consists of user-contributed packages, such as D++!
+
+If you wish, you may enable coroutine and voice support through USE flags. To do so, using your text editor of choice, add the following line to `/etc/portage/package.use/dpp` (as root):
+
+```
+dev-cpp/dpp voice coro
+```
+> You may choose between voice, coro, or both, just pick and choose!
+
+You will now be able to use D++ by including its library on the command line:
+
+```bash
+g++ mybot.cpp -o mybot -ldpp
+```
+
+\include{doc} install_prebuilt_footer.dox