Slight differences for install on Lite version of Pi OS

[Jeff Geerling]

Describe the bug

When I follow the official README, it assumes I'm running full desktop Pi OS, and some things aren't present on the 'Lite' version which require installation before the WebUI will run.

To Reproduce

1. Flash a fresh image of Pi OS 12 'Lite' to a microSD card.
2. Follow the installation instructions in the README.

Expected behavior

Everything works.

Additional context

See: geerlingguy/3d-printing#9 (comment)

A few things I had to do to get things working:

1. apt install git and python3-pip
2. apt install python3-picamera2 --no-install-recommends
3. pip install Flask and Pillow

Note that I dealt with the whole 'externally managed' thing by nuking the EXTERNALLY-MANAGED file so I could just install things on system python. I tried using a venv in the project inside which I tried installing all the dependencies, but it complained about picamera2 not being available, even though I had the system level package python3-picamera2 installed... So I just deleted the virtual Python environment and stomped all over my system install :)