Add a "Write your first test" page to the Get Started section

[oleiade]

What?

As we agreed during our latest meetup, this PR adds a "Write your first test" page to the Getting Started section.

It aims to cover a gap between installing k6, and "Running k6", as we realised we had not introduced users to scripting in the logical flow yet. This PR aims to bridge that gap, and links to a lot of useful resources users might find useful on their onboarding experience with k6.

@heitortsergent this is really a draft on my side, you should feel free to propose whatever you see fit 👍🏻 Also, I've noticed we have some pages in our documentation which might repeat this information, we might want to also reorganize/regroup it somehow 🙇🏻

Checklist

• I have used a meaningful title for the PR.
• I have described the changes I've made in the "What?" section above.
• I have performed a self-review of my changes.
• I have run the npm start command locally and verified that the changes look good.
• I have made my changes in the docs/sources/next folder of the documentation.