docs: remove yarn references and pin jupyterlab

Author: Konstantin Taletskiy

{{cookiecutter.github_project_name}}/README.md

@@ -25,7 +25,7 @@ jupyter nbextension enable --py [--sys-prefix|--user|--system] {{ cookiecutter.p
 
 Create a dev environment:
 ```bash
-conda create -n {{ cookiecutter.python_package_name }}-dev -c conda-forge nodejs yarn python jupyterlab
+conda create -n {{ cookiecutter.python_package_name }}-dev -c conda-forge nodejs python jupyterlab=3.6.7
 conda activate {{ cookiecutter.python_package_name }}-dev
 ```
 
@@ -39,7 +39,7 @@ notebook / lab frontend. For lab, this is done by the command:
 
 ```
 jupyter labextension develop --overwrite .
-yarn run build
+jlpm run build
 ```
 
 For classic notebook, you need to run:
@@ -61,7 +61,7 @@ terminals to watch for changes in the extension's source and automatically rebui
 
 ```bash
 # Watch the source directory in one terminal, automatically rebuilding when needed
-yarn run watch
+jlpm run watch
 # Run JupyterLab in another terminal
 jupyter lab
 ```