Docusaurus 3.0.0

[mikestreety]

Version 3 of Docusaurus got released on 31st October: https://docusaurus.io/blog/releases/3.0

Happy to upgrade the framework which we can, except for one dependency

Ideally we would stick with @easyops-cn/docusaurus-search-local as it is the best search I've found for Docusaurus that can be used on internal, private content - but it needs updating.

There is an issue for it: easyops-cn/docusaurus-search-local#345