[docs] Make it more obvious that mlir_venv isn't automatically activated (#3932)

- i.e. VSCode looks for `.venv` and will automatically activate the
environment
- the virtual environment in this particular doc is `mlir_venv`, so
activation must be manually managed by the developer

Author: bjacobgordon

docs/development.md

@@ -95,6 +95,7 @@ sudo apt install clang ccache lld
 
 #### Configure for Building
 
+1. **If you haven't already**, [activate the Python environment](#set-up-the-python-environment)
 1. Choose command relevant to LLVM setup:
     1. **If you want the more straightforward option**, run the "in-tree" setup: