docs: use python's virtual environment

Debian 12 does not allow simply to run python3 -m pip install
So we should create here venv and install qllr there.

docs/install.md

@@ -3,14 +3,19 @@
 ## Installing and configuring QLLR
 ```
 # base apps
-sudo apt-get install python3 python3-pip postgresql git
+sudo apt-get install python3 python3-pip python3-venv postgresql git
 
 # psycopg2 build prerequires
 sudo apt-get install python3-dev libpq-dev gcc
 
 # install qllr
 git clone https://github.com/em92/quakelive-local-ratings
 cd ./quakelive-local-ratings
+
+# create virtual environment
+python3 -m venv venv
+source venv/bin/activate
+
 python3 -m pip install -r requirements.txt
 ```