Update Gentoo install documentation (#3222)

* docs/install.md: Update Gentoo documentation

Signed-off-by: Andrei Horodniceanu <a.horodniceanu@proton.me>

* contrib: Remove Gentoo ebuild

Drop the Gentoo ebuild in here in favor of using the one in the Gentoo
dlang overlay, since the latter better follows the Gentoo standards.

Signed-off-by: Andrei Horodniceanu <a.horodniceanu@proton.me>

* Update allow.txt

* Add 'eselect' and 'onlydeps'

---------

Signed-off-by: Andrei Horodniceanu <a.horodniceanu@proton.me>
Co-authored-by: abraunegg <alex.braunegg@gmail.com>

contrib/gentoo/onedrive-2.5.5.ebuild

@@ -1,31 +0,0 @@
-# Copyright 1999-2018 Gentoo Foundation
-# Distributed under the terms of the GNU General Public License v2
-
-EAPI=6
-
-DESCRIPTION="Onedrive sync client for Linux"
-HOMEPAGE="https://github.com/abraunegg/onedrive"
-SRC_URI="https://github.com/abraunegg/onedrive/archive/v${PV}.tar.gz -> ${P}.tar.gz"
-
-LICENSE="GPL-3"
-SLOT="0"
-KEYWORDS="~amd64 ~x86"
-IUSE=""
-
-DEPEND="
-	>=dev-lang/dmd-2.081.1
-	dev-db/sqlite
-"
-
-RDEPEND="${DEPEND}
-	net-misc/curl
-	"
-src_prepare() {
-	default
-	# Copy line 38 to 44 as systemd path needs to be created in portage sandbox
-	# Update the makefile so that it doesnt use git commands to get the version during build.
-	sed -i -e "38h; 44p; 44x" \
-		-e "s/version:.*/version:/" \
-		-e "\$s/.*/\techo v${PV} > version/" \
-		Makefile
-}