[V3 Docs] Add install guides to the documentation (#1005)

* [Docs] add install guides to the documentation

* formatting changes

docs/install_centos.rst

@@ -0,0 +1,40 @@
+.. centos install guide
+
+==========================
+Installing Red on CentOS 7
+==========================
+
+---------------------------
+Installing pre-requirements
+---------------------------
+
+.. code-block:: none
+
+    yum -y groupinstall development
+    yum -y install https://centos7.iuscommunity.org/ius-release.rpm
+    yum -y install yum-utils wget which python35u python35u-pip python35u-devel openssl-devel libffi-devel git opus-devel
+    sh -c "$(wget https://gist.githubusercontent.com/mustafaturan/7053900/raw/27f4c8bad3ee2bb0027a1a52dc8501bf1e53b270/latest-ffmpeg-centos6.sh -O -)"
+
+--------------
+Installing Red
+--------------
+
+:code:`pip3 install red-discordbot[voice]`
+
+----------------------
+Setting up an instance
+----------------------
+
+Run :code:`redbot-setup` and follow the prompts. It will ask first for where you want to
+store the data (the default is :code:`~/.local/share/Red-DiscordBot`) and will then ask
+for confirmation of that selection. Next, it will ask you to choose your storage backend
+(the default here is JSON). It will then ask for a name for your instance. This can be
+anything as long as it does not contain spaces; however, keep in mind that this is the
+name you will use to run your bot, and so it should be something you can remember.
+
+-----------
+Running Red
+-----------
+
+Run :code:`redbot <your instance name>` and run through the initial setup. This will ask for
+your token and a prefix.