docs(site-showcase): Add LANDR website to site showcase

[robertcoopercode]

At work, we built out our marketing website using Gatsby and I thought it would be cool to list it on the Gatsby site showcase. The website is https://www.landr.com/en/.

[LekoArts]

Thank you for sharing 👍 Beautiful site

[gatsbot]

Holy buckets, @robertcoopercode — we just merged your PR to Gatsby! 💪💜

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