docs: add showcase section to the contributing guide

[HiDeoo]

What kind of changes does this PR include?

• Changes or translations of Starlight docs site content
• Something else!

Description

This PR adds a new section to the contributing guide, which explains how to add a showcase entry to the Starlight docs site which is something that was never properly documented before.

I've also added a link to this new section in docs/src/content/docs/resources/showcase.mdx so that it's easier to find.

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Latest commit: 92a31a8

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Name	Status	Preview	Updated (UTC)
starlight	✅ Ready (Inspect)	Visit Preview	Mar 28, 2024 0:34am

[at-the-vr]

Great PR, solves a lotta nits with showcase PRs. I thought of a "simple edge-case" regardless its LGTM 👍

[at-the-vr]

I can imagine someone new to showcase-sites trying to find Astro Docs so that they can place their site underneath it 😅 despite 3.1 instruction clearly stating it should be appended at the end

[at-the-vr]

honestly this insight can be overlooked because how trivial it might be in this early stages of Starlight

[sarah11918]

Helped edit on Talking and Doc'ing!

[casungo]

LGTM!

[HiDeoo]

Thanks everyone for your help 🙌