Switch to mkdocs-materialx

The project mkdocs-material is in maintenance-only mode and their
successor is not ready yet.

Use the modern, refreshed theme and drop the pymdownx.magiclink
extension.

docs/requirements.txt

@@ -1,6 +1,6 @@
 mike~=2.1
-mkdocs-include-markdown-plugin~=7.1
-mkdocs-macros-plugin~=1.3
-mkdocs-material[imaging]~=9.5
-mkdocs-minify-plugin~=0.7
+mkdocs-include-markdown-plugin~=7.2
+mkdocs-macros-plugin~=1.5
+mkdocs-materialx[imaging]~=10.1
+mkdocs-minify-plugin~=0.8
 mkdocs-redirects~=1.2