ARROW-7909: [Website] Add how to install on Red Hat Enterprise Linux

`cut -d: -f5 /etc/system-release-cpe` includes only major version on CentOS but it includes both major and minor versions on Red Hat Enterprise Linux.

Closes #46 from taekeharkema/install_url and squashes the following commits:

127f9ed55 <Sutou Kouhei> Fix location
e1f80acdf <Sutou Kouhei> Install epel-release explicitly on RHEL
f93e05cc9 <taekeharkema> Update download URL for Centos 6/7

Lead-authored-by: taekeharkema <32459807+taekeharkema@users.noreply.github.com>
Co-authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>
diff --git a/install.md b/install.md
index 23ae5f7..fc97793 100644
--- a/install.md
+++ b/install.md
@@ -71,6 +71,9 @@
 * CentOS 6
 * CentOS 7
 * CentOS 8
+* Red Hat Enterprise Linux 6
+* Red Hat Enterprise Linux 7
+* Red Hat Enterprise Linux 8
 * Amazon Linux 2
 
 Debian GNU/Linux and Ubuntu:
@@ -98,10 +101,11 @@
 sudo apt install -y -V libparquet-glib-dev # For Apache Parquet GLib (C)
 ```
 
-CentOS 8:
+CentOS 8 and Red Hat Enterprise Linux 8:
 
 ```shell
-sudo dnf install -y https://apache.bintray.com/arrow/centos/$(cut -d: -f5 /etc/system-release-cpe)/apache-arrow-release-latest.rpm
+sudo dnf install -y epel-release || sudo dnf install -y https://dl.fedoraproject.org/pub/epel/epel-release-latest-$(cut -d: -f5 /etc/system-release-cpe | cut -d. -f1).noarch.rpm
+sudo dnf install -y https://apache.bintray.com/arrow/centos/$(cut -d: -f5 /etc/system-release-cpe | cut -d. -f1)/apache-arrow-release-latest.rpm
 sudo dnf install -y --enablerepo=epel --enablerepo=PowerTools arrow-devel # For C++
 sudo dnf install -y --enablerepo=epel --enablerepo=PowerTools arrow-glib-devel # For GLib (C)
 sudo dnf install -y --enablerepo=epel --enablerepo=PowerTools arrow-dataset-devel # For Arrow Dataset C++
@@ -109,10 +113,11 @@
 sudo dnf install -y --enablerepo=epel --enablerepo=PowerTools parquet-glib-devel # For Parquet GLib (C)
 ```
 
-CentOS 6 and 7:
+CentOS 6, CentOS 7, Red Hat Enterprise Linux 6 and Red Hat Enterprise Linux 7:
 
 ```shell
-sudo yum install -y https://apache.bintray.com/arrow/centos/$(cut -d: -f5 /etc/system-release-cpe)/apache-arrow-release-latest.rpm
+sudo yum install -y epel-release || sudo yum install -y https://dl.fedoraproject.org/pub/epel/epel-release-latest-$(cut -d: -f5 /etc/system-release-cpe | cut -d. -f1).noarch.rpm
+sudo yum install -y https://apache.bintray.com/arrow/centos/$(cut -d: -f5 /etc/system-release-cpe | cut -d. -f1)/apache-arrow-release-latest.rpm
 sudo yum install -y --enablerepo=epel arrow-devel # For C++
 sudo yum install -y --enablerepo=epel arrow-glib-devel # For GLib (C)
 sudo yum install -y --enablerepo=epel arrow-dataset-devel # For Arrow Dataset C++