chore: Add util to build our llms.txt and llms-full.txt files

[hangfei]

Add a script to build the llms and llms-full txt files.

1. both files will be in markdown by convention
2. keep only python code samples (can add java support later)
3. will keep these two files in this repo so it's easier to find.
4. later, we can add this as part of our workflow CI/CD

Token count:
llms.txt tokens: 1523
llms-full.txt tokens: 240465