[Docs] Restructure venv instructions (put them in install guides) (#3495)

* docs: restructure venv instructions

* docs: add a warning about PowerShell usage

docs/about_venv.rst

@@ -0,0 +1,39 @@
+.. _about-venvs:
+
+==========================
+About Virtual Environments
+==========================
+Creating a virtual environment is really easy and usually prevents many common installation
+problems.
+
+**What Are Virtual Environments For?**
+
+Virtual environments allow you to isolate Red's library dependencies, cog dependencies and python
+binaries from the rest of your system. There is no performance overhead to using virtual environment
+and it saves you from a lot of troubles during setup. It also makes sure Red and its dependencies
+are installed to a predictable location which makes uninstalling Red as simple as removing a single folder,
+without worrying about losing your data or other things on your system becoming broken.
+
+
+--------------------------------------------
+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.