docs: add Flarum to showcase

[datlechin]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.

Motivation

Request for adding my site docs.flarum.org to showcase.
I have read the contributing guidelines.

Test Plan

N/A

Test links

N/A

Related issues/PRs

N/A

[netlify]

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 52	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 84	🟢 99	🟢 100	🟢 100	🟢 90	Report

[Josh-Cena]

Ooops, merged too fast, I didn't get the chance to format the file & optimize the image💀

Not a big deal, I'll follow-up. My heart aches every time a 300KB image goes into git history