newsletter: Generation of descriptive titles

The TLDR newsletter design includes a preview of the newsletter content in its title. These titles provide a brief glimpse of the key topics or discussions covered in the newsletter.

[Link to designs](https://www.figma.com/design/sJiEDFk7eor5BLLCVqxhdR/Bitcoin-TLDR-New?node-id=1056-20456&t=3FqM1WpsP3eMzGnI-1)

Currently, there is no process in place to automatically generate these title descriptions as part of the weekly newsletter generation process.

#### Solution

We need to update the `generate_weekly_newsletter_json.py` script to include the generation of these title descriptions.
Specifically:
1. **Write a new prompt** that uses the newsletter content as input to generate a concise and relevant title description.
2. Integrate this prompt into the existing workflow so that the title description is generated alongside the rest of the newsletter content.