Add working environment.yml

[talmo]

This PR adds an environment.yml that produces a working conda environment across platforms and corresponding instructions in the README.

The only non-conda package that is used is for PySide6 since QtWebEngine is not yet available for Qt 6.x on conda-forge (conda-forge/qt-webengine-feedstock#46).

I didn't add anything to the docs in case you guys wanted to hold off on that before testing.

[anshu957]

Tested on M1 chip with python version --3.10.15:

• Opening GUI app
• training the classifier
• labeling the frames

All OK ✅

[SkepticRaven]

There's some issues with getting libxcb to link up for the linux conda environment, but I think that's more of a headless or system/conda path issue which we can ignore for now...