Add lastmod to XML sitemap

[jdevalk]

Please give us a description of the issue

The XML sitemap for developer. doesn't have a lastmod value because Docusaurus doesn't support that. I came across this solution for adding lastmod that I thought was smart, I'd suggest implementing it.

• I've read and understood the contribution guidelines.
• I've searched for any related issues and avoided creating a duplicate issue.

[mhkuu]

Thanks a lot for the suggestion! 🙌 Implemented through #331.