Documentation improvements

[jgchn]

Is your feature request related to a problem? Please describe.
Some parts of the documentation can be updated. Creating this issue to track the areas I think can be improved. Happy to open a PR for this!

Describe the solution you'd like

• Bullet point list for introductory paragraphs ("PDL provides the following features...") (link) to improve readability
• The "data" file mentioned in the Type Checking section is incorrect. It is linking to data.yaml, not gen-data.yaml.
• Bullet point list for Parameter defaults for watsonx Granite models section to improve readability

Additional context
Would be great if the Contributing page can include instructions on how to get started. Here are some of the steps I followed.

1. Fork https://github.com/IBM/prompt-declaration-language
2. Clone the fork
3. Run the following commands to set up dev environment

python -m venv .venv 
source .venv/bin/activate
pip install e .

4. Run some examples. For some reason, setting the env variable REPLICATE_API_TOKEN did not work for me. However, REPLICATE_API_KEY did. I wonder if it has to do with how LiteLLM expecting the latter env var for Replicate? Not entirely sure why.

pdl examples/hello/hello.pdl