Improve sitemaps design & performance

[remibetin]

Preview link: https://deploy-preview-419--wai-website.netlify.app/

• Manage translations sitemaps as a Jekyll collection

Related to :

• https://github.com/w3c/wai-website-theme/pull/88
• https://github.com/w3c/wai-website-data/pull/145
• https://github.com/w3c/wai-about-wai/pull/265

Resolves https://github.com/w3c/wai-website/issues/403 Resolves https://github.com/w3c/wai-translations/issues/122

[netlify[bot]]

✅ Deploy Preview for wai-website canceled.

Name	Link
🔨 Latest commit	e2e79390c59846b2a48fae333e7fbdaf8eb75433
🔍 Latest deploy log	https://app.netlify.com/sites/wai-website/deploys/6633e2682acb5e0008d9bf45