docs: add Gotenberg to showcase

[gulien]

Motivation

I've spent a lot of time this week building the new website for Gotenberg, and it has been a blast with Docusaurus v2 😄

This MR adds the Gotenberg website to showcases.

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[netlify]

✔️ [V2]
Built without sensitive environment variables

🔨 Explore the source changes: bb6fe9f

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/61227a55cf21ca0007da6371

😎 Browse the preview: https://deploy-preview-5405--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	98
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5405--docusaurus-2.netlify.app/

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[lex111]

@gulien your site looks pretty good, thanks for choosing us!

[slorber]

Thanks 👍 :)