Upgrade mkdocs-material to 9.x?

[Roiocam]

paradox-material-theme based on the old version of Material for MkDocs, Although it meets most of our needs, it is still unavailable (low performance) on some hotspot functions (such as search). See: https://github.com/apache/incubator-pekko/issues/1097

But mkdocs-material seemed to change there build after 6.x? It doesn't seem to be a simple thing for us, because it need to rewrite a lot of code.

[mdedetrich]

Even though its a big change I am personally for it, @mkurz @jonas thoughts?

[mkurz]

I have no objections.

[samueleresca]

This would be amazing. Note that, it will require some changes downstream in apache/incubator-pekko-sbt-paradox and apache/incubator-pekko-site)

[mkurz]

Pull requests welcome.