feat(sitemap): add support for "lastmod"

[slorber]

Motivation

Fix #2604

Add new sitemap options:

{
  lastmod: null | 'date' | 'datetime'
  priority: null,
  changefreq: null,
}

• Add a new sitemap option lastmod that permits to output <lastmod> tags in the sitemap, formatted as YYYY-MM-DD date, or ISO datetime
• Allow to disable useless <changefreq> and <priority> sitemap tags (Google doesn't use them) with null values

Important notes:

• lastmod only works for routes backed and requires plugin authors to add new route metadata when calling addRoute()
• All our official content plugins provide this route metadata
• If plugins allow to provide a last updated time as front matter, then it's possible for the sitemap plugin to use this explicit date in priority. Otherwise, a timestamp will be inferred from the Git history of the content source file.
• We are able to output a <lastmod> tag for React page components as well. However it will only use the React page source Git history, and will not check history of its dependencies.
• Not all routes will have a <lastmod> tag. If a third-party plugin does not provide the new metadata, the sitemap can't infer a proper value. According to Google recommendations, it's only relevant to use <lastmod> tags for routes backed by content, and not routes aggregating multiple documents (tags, blog pagination, blog archive, doc autogenerated index...)

Test Plan

Unit tests + dogfood on our website

Test links

• Website sitemap: https://deploy-preview-9954--docusaurus-2.netlify.app/sitemap.xml
• Sitemap options doc: https://deploy-preview-9954--docusaurus-2.netlify.app/docs/api/plugins/@docusaurus/plugin-sitemap/
• RouteMetadata doc: https://deploy-preview-9954--docusaurus-2.netlify.app/docs/api/plugin-methods/lifecycle-apis/#addRoute

Related issues/PRs

Supersed PR #9234

[netlify]

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🔴 46	🟢 98	🟢 96	🟢 100	🟠 88	Report
/docs/installation	🟠 59	🟢 96	🟢 100	🟢 100	🟠 88	Report
/docs/category/getting-started	🟠 76	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog	🟠 69	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 59	🟢 96	🟢 100	🟢 100	🟠 88	Report
/blog/tags/release	🟠 71	🟢 100	🟢 100	🟠 80	🟠 88	Report
/blog/tags	🟠 76	🟢 100	🟢 100	🟢 90	🟠 88	Report

[github-actions]

Size Change: +45 B (0%)

Total Size: 992 kB

Filename	Size	Change
website/build/assets/js/main.********.js	765 kB	+45 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	75.4 kB
website/build/assets/css/styles.********.css	114 kB
website/build/index.html	37.9 kB

_{compressed-size-action}

[johnnyreilly]

This is excellent work; I like that you're looking to make the default sitemap include lastmod by default with Docusaurus v4