docs: mention Netlify ignore build setting

[slorber]

Motivation

Add some useful doc for v1 sites that upgraded and using Netlify, as it affected some FB Docusaurus sites

[netlify]

✔️ [V2]

🔨 Explore the source changes: 3a7cb12

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/6132588daea0e80007c71c99

😎 Browse the preview: https://deploy-preview-5486--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

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

Lighthouse ran on https://deploy-preview-5486--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 826 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	36.7 kB
website/build/assets/css/styles.********.css	93.9 kB
website/build/assets/js/main.********.js	414 kB
website/build/blog/2017/12/14/introducing-docusaurus/index.html	67 kB
website/build/blog/index.html	38.1 kB
website/build/docs/index.html	44.6 kB
website/build/docs/installation/index.html	52.8 kB
website/build/index.html	30.8 kB
website/build/tests/docs/index.html	25.4 kB
website/build/tests/docs/standalone/index.html	23 kB

_{compressed-size-action}

[netlify]

✔️ [V2]

🔨 Explore the source changes: 09ae0d8

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/613258e84e8a0f0008bd962c

😎 Browse the preview: https://deploy-preview-5486--docusaurus-2.netlify.app

[JoelMarcey]

Approved even after the merge 😏