Improve docs

[ryw]

Summary

Enhanced quickstart guide with clearer prerequisites and better guidance for first-time users:

• Added introductory sentence and "Before you start" section listing required access and optional integrations
• Improved "Create a task" step with guidance to start with one repository and narrow scope for easier review
• Added "Good first tasks" section with basic and advanced example prompts to help users understand task complexity
• Updated changelog metadata with description field for better SEO

These changes make the documentation more accessible and actionable for new users getting started with Tembo.

Want tembo to make any changes? Add a review or comment with @tembo and i'll get back to work!

[tembo]

Requesting review from @benja who has experience with the following files modified in this PR:

• quickstart.mdx

[ryw]

@tembo !review