Improved development environment setup instructions (#5876)

Author: sisp

docs/customization.md

@@ -235,20 +235,38 @@ directly in the source of the theme and recompile it.
 
 ### Environment setup
 
-In order to start development on Material for MkDocs, a [Node.js] version of
-at least 18 is required. First, clone the repository:
+First, clone the repository:
 
 ```
 git clone https://github.com/squidfunk/mkdocs-material
+cd mkdocs-material
 ```
 
-Next, all dependencies need to be installed, which is done with:
+Next, create a new [Python virtual environment][venv] and
+[activate][venv-activate] it:
+
+```
+python -m venv .venv
+source .venv/bin/activate
+```
+
+  [venv]: https://docs.python.org/3/library/venv.html
+  [venv-activate]: https://docs.python.org/3/library/venv.html#how-venvs-work
+
+Then, install all Python dependencies:
 
 ```
-cd mkdocs-material
 pip install -e .
 pip install mkdocs-minify-plugin
 pip install mkdocs-redirects
+pip install nodeenv
+```
+
+Finally, install the [Node.js] LTS version into the Python virtual environment
+and install all Node.js dependencies:
+
+```
+python -m nodeenv -p -n lts
 npm install
 ```