Demo Decode Colab in MaxText

[A9isha]

Description

In this PR, we are refreshing/fixing the previous MaxText/scratch_code/demo_from_pretrained.ipynb and moving it out of scratch_codeto MaxText/examples/demo_decoding.ipynb. Also for documentation purpose, we update the getting_started/First_run.md. This notebook demonstates decoding in MaxText. In the follow up PR, we will add pretraining demo notebook.

FIXES: b/435926234

Tests

Locally ran the notebook on a v5p-8

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.

[richjames0]

Thanks! Should add something to readme at some point (as you were probably planning!)