Fix the quickstart notebook

[alan-cooney]

Fix the quickstart notebook so that it will work with version 4.1.

• Remove the Acme extra requirement reverb which longer exists, and isn't needed (as dm-reverb is installed as part of the TensorFlow extra requirements).
• Add the Acme extra requirement jax, which is required.
• Add gym's extra classic_control dependencies, and Imageio's extra ffmpeg dependencies, which are required to render the video.
• Add the output video file to .gitignore.

Note Colab has unfortunately also made some formatting changes to the raw notebook code (replacing html-encoded angle brackets e.g. \u003ca with pure < brackets, removing some colab metadata, and setting execution count to null rather than 0). Given it does this by default now (and Jupyter Lab does the same) I've left these changes in.

I have checked this with both Colab and Jupyter Lab, and it works on both (successfully training). It is also backwards compatible with v. 4.0 (so it won't break anything if merged now, before 4.1 is released).