[Docs] Add virtualenv guide and compress install guides (#2029)

* [Docs] Add virtualenv guide and compress install guides

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* [Docs] Better cross-referencing

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Fix pyenv-installer link

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Use sudo -e instead of sudo nano

* Add note about launcher for linux/mac

* Include launcher notes in Windows guide

* Add missing colon

docs/venv_guide.rst

@@ -0,0 +1,132 @@
+.. _installing-in-virtual-environment:
+
+=======================================
+Installing Red in a Virtual Environment
+=======================================
+Virtual environments allow you to isolate red's library dependencies, cog dependencies and python
+binaries from the rest of your system. It is strongly recommended you use this if you use python
+for more than just Red.
+
+.. _using-venv:
+
+--------------
+Using ``venv``
+--------------
+This is the quickest way to get your virtual environment up and running, as `venv` is shipped with
+python.
+
+First, choose a directory where you would like to create your virtual environment. It's a good idea
+to keep it in a location which is easy to type out the path to. From now, we'll call it
+``path/to/venv/`` (or ``path\to\venv\`` on Windows).
+
+~~~~~~~~~~~~~~~~~~~~~~~~
+``venv`` on Linux or Mac
+~~~~~~~~~~~~~~~~~~~~~~~~
+Create your virtual environment with the following command::
+
+    python3 -m venv path/to/venv/
+
+And activate it with the following command::
+
+    source path/to/venv/bin/activate
+
+.. important::
+
+    You must activate the virtual environment with the above command every time you open a new
+    shell to run, install or update Red.
+
+Continue reading `below <after-activating-virtual-environment>`.
+
+~~~~~~~~~~~~~~~~~~~
+``venv`` on Windows
+~~~~~~~~~~~~~~~~~~~
+Create your virtual environment with the following command::
+
+    python -m venv path\to\venv\
+
+And activate it with the following command::
+
+    path\to\venv\Scripts\activate.bat
+
+.. important::
+
+    You must activate the virtual environment with the above command every time you open a new
+    Command Prompt to run, install or update Red.
+
+Continue reading `below <after-activating-virtual-environment>`.
+
+.. _using-pyenv-virtualenv:
+
+--------------------------
+Using ``pyenv virtualenv``
+--------------------------
+
+.. note::
+
+    This is for non-Windows users only.
+
+Using ``pyenv virtualenv`` saves you the headache of remembering where you installed your virtual
+environments. If you haven't already, install pyenv with `pyenv-installer`_.
+
+First, ensure your pyenv interpreter is set to python 3.6 or later with the following command::
+
+    pyenv version
+
+Now, create a virtual environment with the following command::
+
+    pyenv virtualenv <name>
+
+Replace ``<name>`` with whatever you like. If you forget what you named it, use the command ``pyenv
+versions``.
+
+Now activate your virtualenv with the following command::
+
+    pyenv shell <name>
+
+.. important::
+
+    You must activate the virtual environment with the above command every time you open a new
+    shell to run, install or update Red.
+
+Continue reading `below <after-activating-virtual-environment>`.
+
+.. _pyenv-installer: https://github.com/pyenv/pyenv-installer/blob/master/README.rst
+
+----
+
+.. _after-activating-virtual-environment:
+
+Once activated, your ``PATH`` environment variable will be modified to use the virtual
+environment's python executables, as well as other executables like ``pip``.
+
+From here, install Red using the commands listed on your installation guide (`Windows
+<installing-red-windows>` or `Non-Windows <installing-red-linux-mac>`).
+
+.. note::
+
+    The alternative to activating the virtual environment each time you open a new shell is to
+    provide the full path to the executable. This will automatically use the virtual environment's
+    python interpreter and installed libraries.
+
+--------------------------------------------
+Virtual Environments with Multiple Instances
+--------------------------------------------
+If you are running multiple instances of Red on the same machine, you have the option of either
+using the same virtual environment for all of them, or creating separate ones.
+
+.. note::
+
+    This only applies for multiple instances of V3. If you are running a V2 instance as well,
+    You **must** use separate virtual environments.
+
+The advantages of using a *single* virtual environment for all of your V3 instances are:
+
+- When updating Red, you will only need to update it once for all instances (however you will still need to restart all instances for the changes to take effect)
+- It will save space on your hard drive
+
+On the other hand, you may wish to update each of your instances individually.
+
+.. important::
+
+    Windows users with multiple instances should create *separate* virtual environments, as
+    updating multiple running instances at once is likely to cause errors.