[readme] Improve getting started section

[adnanrahic]

Improve getting started section to include:

• Hosted Tracetest (free to get started!) - The fastest and most reliable way to get started with Tracetest is by signing up for free...
• Open-source self-hosted - Deploy a developer/hobby instance locally or with Docker/Kubernetes
• Enterprise self-hosted (coming soon) - Same as hosted Tracetest but in your own env

[adnanrahic]

I like this way of structuring the getting started section: https://github.com/highlight/highlight?tab=readme-ov-file#get-started