misc: add script to keep starters branch/repos up-to-date

[slorber]

Motivation

For Netlify deploy button, subfolders such as examples/classic won't work and we must have a real starter repository.
See jamstack/jamstack.org#609

This little automation will permit to keep some starter repo/branch up-to-date:

• https://github.com/facebook/docusaurus/tree/starter
• https://github.com/slorber/docusaurus-starter

In the future having an i18n starter could also be helpful for users to inspect it easily.

[netlify]

✔️ [V2]

🔨 Explore the source changes: 5d1e39d

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60f98b89748dd6000746dc34

😎 Browse the preview: https://deploy-preview-5206--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	69
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5206--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 851 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	58.8 kB
website/build/assets/css/styles.********.css	89.1 kB
website/build/assets/js/main.********.js	460 kB
website/build/blog/2017/12/14/introducing-docusaurus/index.html	62.3 kB
website/build/blog/index.html	26.3 kB
website/build/docs/index.html	40.2 kB
website/build/docs/installation/index.html	47.8 kB
website/build/index.html	27.3 kB
website/build/tests/docs/index.html	20.3 kB
website/build/tests/docs/standalone/index.html	18.8 kB

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