Put `!environments` and `!venv` in a tag group

[TizzySaurus]

We currently have two tags, which both refer to virtual environments. These are !environments and !venv.

However, these two tags serve difference purposes (what a venv is and how to use one, respectively), so deleting one isn't really an option and they're too long to merge into a single tag.

The proposed solution for this (suggested by @Akarys42 which I agree with), is to wait for tag groups (#1663) is merged and then put these within a virtual environments tag.